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tion to t; join t w, and we have the other or downward side of the roof; the lines x and y of the roof (in the building itself) are parallel to st, and have the same VP, namely, VP3.

Our next problems will have especial reference to two inclinations, and will require very close attention.

READINGS IN FRENCH.-XI. FEDORA.

SECTION III.

"JE suis," dit-il, "un commerçant de Moscou;1 j'avais entrepris (a) un voyage à Varsovie. Avant de m'éloigner (b), j'ai voulu aller visiter un gentilhomme que je connais, et qui demeure à quelque distance; armé d'un fusil, je me suis rendu à pied à son château, où je me suis attardé. La neige tombait à gros flocons, je me suis (c) égaré ; je cherchais en vain ma route, quand je vis venir à moi deux hommes à qui je m'empressai de demander quelques renseignements. Je n'avais aucune défiance, et j'attendais tranquillement leur réponse, quand tout à coup ces deux scélérats, se précipitant sur moi, me terrassèrent et me dépouillèrent du peu d'argent que j'avais. Je poussai un cri; c'est alors que l'un d'eux tira sur moi un coup de pistolet, car ils

voulaient me tuer."

9

Pendant le récit de l'étranger, Fodora avait eu l'œil constamment fixé sur lui. Il (d) lui semblait retrouver sur son visage des traits connus, sans se rappeler où elle l'avait vu; toutefois, son cœur battait avec violence, un sentiment 10 irrésistible l'at tirait vers lui. Polowski pria son hôte de lui donner quelques

détails sur l'incendie de Moscou.

L'étranger parut éprouver quelque répugnance12 à satisfaire à cette demande; néanmoins, en réfléchissant au service qu'il avait reçu, il n'osa (e) refuser. En décrivant le triste spectacle, de ce vaste incendie, sa voix trahissait de vives émotions.13 Mais quand il commença à raconter ses propres malheurs, il versa des larmes abondantes; et poussant un profond soupir,

il dit :

"Hélas! ce terrible incendie ne m'a pas seulement enlevé une grande partie de ma fortune,15 mais encore ce qui faisait(f) tout notre bonheur, ma fille bien-aimée. Tandis qu'au milieu du désordre affreux excité par l'épouvantable catastrophe, nous cherchions, ma femme et moi,16 à soustraire à la voracité des flammes nos objets les plus précieux, nous perdîmes notre enfant, alors dans sa sixième année; sa bonne l'avait prise avec elle afin de la conduire dans la maison d'un ami1 qui demeurait dans une rue écartée, où le feu ne sévissait pas encore. Mais ni la bonne ni l'enfant n'ont reparu, et depuis cet événement, 18 toutes nos informations ont été stériles. Probablement, quelque édifice, en croulant, les aura englouties sous ses décombres."

À ces mots Fodora, qui avait écouté avidement toutes les particularités de ce récit, ne put (g) contenir davantage les émotions 19 qu'il avait excitées en elle. Elle se précipita au cou de l'étranger, en s'écriant

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Après les premiers épanchements, Fodora raconta à son tour l'espèce de succession de miracles' auxquels elle devait (a) son salut. La bonne vivandière ne fut point oubliée dans ce récit: mais avec quelle sensibilité, quelle touchante expression de reconnaissance, elle exposa tout ce qu'elle devait à la bienveil lance du seigneur polonais qui l'avait recueillie et traitée (b) comme son enfant.

Au bout de quelques jours, la blessure du père de Fodora était guérie. Il dut (c) quitter le château de Polowski, et Fœdora le suivit, non sans assurer à ses bienfaiteurs que sa reconnaissance ne s'éteindrait qu'avec sa vie.

Ils revinrent (d) à Moscou,' où leur retour causa une joyeuse surprise. L'histoire de Fodora se répandit (e). Un jeune seigneur russe, qui occupait un haut grade dans les rangs de l'armée, demanda la jeune fille en mariage et l'épousa.

Dix ans s'étaient écoulés. La Pologne avait proclamé son indépendance, et l'empereur de toutes les Russies, Nicolas, mit une armée en campagne 10 pour comprimer les efforts de cette héroïque nation. On sait (f) l'issue de cette lutte inégale. L'époux de Fodora avait pris part à la campagne;" Fodora l'avait suivi. Sans nous arrêter à rappeler les scènes déchirantes de cette guerre, nous dirons seulement que Varsovie venait d'être emporté d'assaut. Dans ce jour néfaste, des milliers de Polonais et de Russes périrent.13 Vers le soir, tous les officiers supérieurs de l'armée triomphante allèrent visiter le champ de bataille, où gisaient (g) pêle-mêle les cadavres des vainqueurs et des vaincus.

Là, gémissaient confondus parmi les morts, un nombre infini de blessés.15 Poussé par la charité, ému de compassion pour le destin de ceux à qui la fortune avait été contraire, l'époux de Fodora fit transporter dans les hôpitaux et les ambulances, ceux à qui il restait encore un souffle de vie. Après ces soins pieux, il allait s'éloigner de ce lieu de désolation, lorsque parmi plusieurs cadavres qu'on allait recouvrir de terre, il aperçut un officier polonais de haut rang et tout chamarré de croix et de décorations. Il crut remarquer en lui quelques signes de vie,15 et le fit transporter dans la maison 19 même où était Fadora. Là tous les soins nécessaires lui furent prodigués ; 20 et, peu à peu, sortant de sa léthargie, l'officier polonais rouvrit les yeux.

Fodora était assise (h) au chevet de son lit.21 Tout à coup elle poussa un cri, elle avait reconnu Polowski.

Polowski, rétabli de ses blessures, n'avait échappé à un péril, que pour retomber dans un danger plus terrible encore.22 Son nom fut porté sur la liste des proscrits.23 Quand Fodora l'apprit, elle se rendit immédiatement près de l'empereur; elle embrassa ses genoux, et demanda sa grâce, et Nicolas attendri, prononça le pardon de Polowski.

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KEY TO EXERCISES IN LESSONS IN FRENCH.

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EXERCISE 136 (Vol. II., page 298).

8.

19.

1. M. votre frère a-t-il pris garde de gâter son chapeau ? 2. Il a pris garde de le gâter, il n'en a qu'un. 3. Allez parler à Mlle. votre sœur, elle vous appelle. 4. Ne voulez-vous pas prendre une tasse de the? 5. Je viens de prendre le thé. 6. Qu'avez-vous dit à votre petite fille? 7. Je lui ai dit de prendre garde de déchirer sa robe. Prenons garde de déchirer ce livre. 9. Mon fils vient de l'apporter. 10. A-t-il pris le thé? 11. Il n'a pas encore pris le thé, il est trop tôt. 12. À quelle heure prenez-vous le thé chez vous. 13. Nous prenons le thé à six heures. 14. Prenez-vous du thé ou du café à votre dejeûner? 15. Nous prenons du café. 16. Votre courrier a-t-il pris les devants? 17. Il n'a pu prendre les devants. 18. Quel parti avez-vous pris? J'ai pris le parti d'étudier ma leçon. 20. Avez-vous pris garde de déchirer vos livres? 21. J'ai pris garde de les tacher. 22. Quel parti votre frère a-t-il pris? 23. Il a pris le parti de se taire. 24. Avezvons pris mon parti? 25. J'ai pris le parti de mon frère. 26. Avezvous raison de prendre son parti? 27. J'ai raison de prendre son parti, parcequ'il a raison. 28. N'avez-vous pas peur de prendre son parti? 29. Je n'ai pas peur de prendre son parti. 30. Prendrez-vous le parti de votre sceur ou le mien? 31. Je prendrai le parti de ma 32. Allez lire votre livre, vous ne savez pas votre leçon. 33. Je sais ma leçon, et je sais aussi que vous êtes mon ami. 34. Allons trouver notre père, il a besoin de nous.

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EXERCISE 137 (Vol. II., page 330).

1. What do you wish us to do? 2. I wish you to pay attention to your studies. 3. Do you not fear that the rain may prevent your gout? 4. We fear very much lest the rain may prevent us from fulling our engagements. 5. Do you doubt his being at home now? 6. I doubt his being there, it is already ten. 7. Do you require him to set out early? 8. I am astonished that he is not yet gone. 9. Do you prefer that I return these bracelets to you? 10. I prefer that you pay me for them. 11. Does your neighbour fear lest his child may go out? 12. He fears lest he fall in the street. 13. Do you not wish that your pupils may obey you? 14. I wish them to obey me, and to obey their professors. 15. Do you not fear lest that mechanic be ill? 16. I fear his being ill, for his workshop is very unhealthy. 17. Do you not regret his being obliged to work? 18. I regret his being obliged to labour above his strength. 19. Do you not wish that he be acquainted with this news? 20. I wish him to be told of it as soon as possible. 21. Does not your father wish you to buy a warehouse? He wishes me to buy a saw-mill. 23. Do you wish me to leave you? 24. I wish you to remain with me. 25. I wish you to start this morning. EXERCISE 138 (Vol. II., page 330).

22.

1. Voulez-vous que je parle à l'artisan ? 2. Je désire que vous lui disiez de venir ici demain matin. 3. Que voulez-vous que je fasse ? 4. Je désire que vous m'apportiez un livre. 5. Ne désirez-vous pas que je lise votre lettre ? 6. Je désire que vous la lisiez et que vous la donniez à mes sœurs. 7. Mlle. votre sœur ne craint-elle pas que la pluie ne l'empêche de sortir? 8. Elle craint que la pluie ne nous empêche de sortir. 9. Doutez-vous que M. votre père soit à la maison à présent? 10. Je doute qu'il y soit. 11. Exigez-vous que je fasse mon travail à présent? 12. Je désire que vous fassiez votre travail avant de sortir. 13. Ne regrettez-vous pas que vous soyez obligé de travailler ? 14. Je ne regrette pas d'être obligé de travailler. 15. Nêtes-vous pas étonné qu'il le sache? 16. Je suis étonné qu'il sache tout. 17. Exigez-vous que je le paie aujourd'hui ? 18. Je désire que vous le payiez demain. 19. Que voulez-vous que je fasse ? 20. Je veux que vous le payiez immédiatement. 21. Craignez-vous que le maitre ne punisse votre fils? 22. Je crains qu'il ne le punisse pas. 23. Que voulez-vous que je dise? 24. Je veux que vous disiez la vérité, 25. M. votre père ne désire-t-il pas que vous achetiez une maison ? 26. Il désire que j'achète un magasin. 27. Désirez-vous que nous vous quittions? 3. Je désire que vous partiez demain. 29. Voulez-vous que je reste avec vous ? 30. Je désire que vous restiez ici. 31. Désirez-vous que je lui dise cette nouvelle ? 32. Je désire que vous la lui disiez. Désirez-vous que vos enfants obéissent à leur instituteur ? 31. Je désire qu'ils lui obéissent.

EXERCISE 139 (Vol. II., page 331).

33.

3.

9.

1. What must I say? 2. You must say what you have heard. Is it not necessary that I finish that history? 4. It is not necessary for you to finish it. 5. Is it not proper for me to satisfy my creditors?! 6. It is proper that you do it. 7. Is it not right that I pay you what I have borrowed from you? 8. It is right that you pay it to me. Can it be that your brother has forgotten his family? 10. It cannot be that he has forgotten it. 11. Is it certain that your brother has forgotten himself to such a degree? 12. It is certain that he has forgotten himself. 13. It is very sad that he has forgotten himself s0. 14. Will you remain until I have put my affairs in order? 15. I shall remain until you have regulated them. 16. Will it not be necessary for me to furnish that family with provisions? 17. It will be neces sary for you to furnish them, provided you have them. 18. Will it not be better that you lend him money, than let him want for necessaries?

19. It will be better that we lend him some. 21. You must carry this linen to my house.

20. What must we do? 22. Is it not time for me

1. Que faut-il que notre ami fasse ? 2. Il faut qu'il reste chez nous jusqu'à mon arrivée. 3. Que faut-il que notre voisin fasse ? 4. Il faut qu'il mette ordre à ses affaires. 5. N'est-il pas juste que vous payiez vos créanciers ? 6. Il est juste que je les paie. 7. Est-il temps que votre petit garçon aille à l'école ? 8. Il est temps qu'il aille à l'école, il est dix heures. 9. Faut-il que j'écrive à votre correspondant aujourd'hui ou demain ? 10. Il faut que vous lui écriviez demain matin. 11. N'est-il pas fàcheux que votre frère ait déchiré sa casquette ? 12. Il est fâcheux qu'il l'ait déchirée. 13. Faut-il que Mme. votre mère finisse sa lettre ? 14. Il n'est pas nécessaire qu'elle la finisse. 15. Est-il certain que M. votre fils ait oublié son argent?

16.

Il est certain qu'il l'a oublié. 17. Il n'est nullement certain qu'il l'ait oublié. 18. Faut-il que vous fournissiez de l'argent à cet artisan? 19. Il faut que je lui en fournisse, il n'en a pas. 20. Quoique vous fassiez, Vous ne réussirez pas. 21. Quoique dise M. votre frère personne ne le croira. 22. Faut-il que je vous écrive? 23. Il faut que vous m'écriviez. 24. Désirez-vous que je sois malade? 25. Je ne désire pas que vous tombiez malade. 26. Exigez-vous que je vous dise cela? 27. Il faut que vous me disiez tout. 28. Désirez-vous que j'aille chez

vous ? 29. Je désire que vous y alliez. 30. Faut-il que je me lève? 31. Il faut que vous vous leviez à l'instant. 32. Faut-il que M. votre frère se couche ? 33. Il faut qu'il se couche à l'instant. 34. Il est temps qu'il se couche, il est minuit.

EXERCISE 141 (Vol. II., page 365).

1. Do you think that this cloth will last long? 2. I think that it will wear well, for it is strong. 3. Do you think that our porter will be long coming back? 4. I think that he will not tarry. 5. Do you wish us to stand? 6. On the contrary, I wish you to sit down. 7. Do you believe that those students can learn five pages by heart in two hours? 8. I believe that it is impossible. 9. Do you hope that our friend may come early? 10. I hope that he will arrive soon. 11. What kind of a decanter must you have? 12. I want one which holds a litre. 13. I have a crystal one which holds two litres. 14. Do you think that that merchant grows rich at your expense ? 15. I know that he grows rich at the expense of others. 16. What parasol do you think of lending me? 17. I think of lending you the best I have. 18. Will the tanner succeed in earning a living? 19. I do not think that he will succeed. 20. Do you think that this money will suffice for your father? 21. I think that it will be sufficient for him. 22. Do you believe that those gentlemen depend upon me? 23. I know that they depend upon you. 21. Do you think that the concert will take place to-day ? 25. I think that it will not take place.

POPULAR EDUCATOR CLASSES. IT is with much pleasure that we recur to the subject of these classes, and place before our readers a communication from the Promoter of the POPULAR EDUCATOR CLASS established at BRISTOL, which, in addition to presenting an example of the general interest awakened by the paper which appeared in Part XIII. (Vol. II., p. 411), affords a notable instance of the success which has attended the movement in that city, and of the comparative facility with which the result was secured. The statement, as conveyed to us by this energetic student,* is so emphatically practical in all its details that we venture to print

it without further comment.

He addresses us as follows:

"Being convinced that the publication in your pages of the result of our efforts to establish a POPULAR EDUCATOR CLASS, can hardly fail to give an impetus to the establishment of similar classes in various parts of the country, I have ventured to send you a curtailed account of the movement in this city; and hope that, by laying before your readers the facility with which we have established these classes, the enthusiasm with which they have been hailed, and the success with which they have been attended, they will be induced to take action, and show their readiness to follow a good example.

"Our first care was to provide a suitable meeting room; which done, we wrote to the Publishers for a large number of circulars and window-bills, with which we were promptly sup

*This gentleman has kindly given us permission to put in direct communication with him any of our students who may desire to address him upon the subject of his successful efforts in the promotion of the BRISTOL CLASS. He will be happy to furnish any further information that may tend to facilitate the establishment of these classes in other towns. Any letters of request reaching us, asking for the name and address of the "Promoter of the Bristol POPULAR EDUCATOR CLASSES," will receive immediate attention.

plied. We thus called together and held our two preliminary meetings, which were attended by about 45 and 120 young men respectively. At these meetings the whole of our arrangements were completed, and it was determined to open the classes. The preliminary business consisted of

"1st. The proposition of the rules.
"2nd. The election of the officers.
"3rd. The choosing of the subjects.
"4th. General arrangements.

Through the assistance of Messrs. Cassell, Petter, and Galpin, to whom on behalf of the classes we beg to tender our sincerest thanks, these preliminary meetings were held without a farthing expense beyond the rent of a meeting room. The classes have excited the deepest interest in every part of the city, and when we announce that 150 names are already on the books, we feel that it would be superfluous to add anything as to their entire success.

"The following is a copy of the rules which we have adopted in our POPULAR EDUCATOR CLASSES:

66 RULES.

"1. That the subjects treated on shall be those embraced by the new edition of CASSELL'S POPULAR EDUCATOR, and be conducted on the principles explained in Part XII., page 411, of that work.

"2. That the affairs of the Society shall be regulated by the following officers (to be eligible for re-election every six months), viz. :-President, Vice-president, Secretary, Treasurer, and four Members of the Classes, the whole constituting a Committee of Management.

"3. That the subjects for study shall be chosen by the votes of the Members from those treated on in the work alluded to in

Rule 1.

"4. That during the space of time allotted to each subject, the Members belonging to the respective Classes shall analyse and review that portion which may have been appointed for study in the interim between the Meetings, submitting any difficulties to the respective leaders.

"5. That no Member shall make any proposition to the Classes, having reference to its management or mode of procedure, unless he shall have previously intimated to the Committee of Management the nature of such proposition, and obtained their sanction to bring it before the Classes.

"6. That the Committee be empowered to introduce any changes in the management of the Classes which may be in their judg-| ment conducive to their increased success and development; such changes including the introduction of new subjects, the removal of the Society's Meeting Room or Rooms, and any changes which shall be found expedient.

"7. That every Member shall be a Subscriber to the new series of CASSELL'S POPULAR EDUCATOR, and that those who shall, subsequently to the printing of these Rules, join the Classes, and who do not subscribe to that serial, shall immediately become subscribers, and further, shall obtain such back parts as the carrying on of the business of the classes shall render it necessary to obtain.

"8. That this Society engage no paid teachers, and be carried on without any expenses beyond the rent of a Meeting Room, and other unavoidable incidental expenses.

"9. That the Committee reserve to themselves the right of discrimination, and be empowered to disallow any of doubtful character to join the Classes, or any person who in their judgment might prove in any way detrimental to their progress.

"10. That any Member conducting himself in an officious or in an improper manner, in any way becoming obnoxious to those present, or not conforming to the whole of the Rules of the Society, be expelled from the Classes.

"11. That the name of any Member absenting himself during the whole of three months, without legitimate and specified cause, shall be erased from the list of Members.

"12. That the Committee be empowered to alter either or the whole of the foregoing Rules, or to create additional Rules, or to effect any changes which may prove essential to the increased success of the Society.

"We are now in full working order; and as it may be useful to many of your readers, I will endeavour very concisely to explain our mode of conducting the classes.

"First, the affairs of the society are regulated by a committee of management, consisting of five officers, viz.: president, vicepresident, secretary, treasurer, and librarian, and four members chosen by the classes. To these the management of the classes is left, with power to effect any changes which, in their judg ment, might tend to increase their success and development.

"Being unable to obtain the use of the room in which we are now holding our meetings oftener than one evening per week, we are compelled to confine ourselves for the present to the study of French, English, and Shorthand; and we have had but comparatively little difficulty in obtaining the gratuitous services of very accomplished masters, or leaders, in these several departments. Their method is simple, yet most effective. For instance, take Lessons in English, No. II. Here the leader would require from the members of his class answers to his questions respecting every part of that lesson. Where the inaccuracy of the replies indicated a haziness of conception, he would first point out the mistakes, and then, by means of a black-board, illustrate the precise meaning and mode of application of all the terms employed. It is positively astonishing how small a portion of time this occupies, and our English leader highly eulogised the admirable manner in which the lessons are presented, and considers them specially adapted for the conducting of such classes.

"In the French class there are about a hundred members, and it may at first sight appear incredible that any master should be able to conduct successfully so large a class together. It is, however, not only practicable, but even simple. Take Lesson II. in Part I. of the new series of this work; it contains several matters of importance. Our French master takes that lesson, and touches, as they appear in order, upon every point. The difficulties, as fast as they appear, by his judicious mode of teaching, vanish. Should any members be in doubt on any point they mention it, and are immediately put right. Afterwards follow the exercises. Each in turn pronounces one of the sentences, and it will be readily understood that the master's continual corrections and repetitions constitute a really good lesson in French pronunciation. He then appoints, for home work, the lesson for the following week, including two English exercises to be translated into French, and two French exercises into English. The members deliver these exercises to the French master enclosed in cloth envelopes, with which they are sup plied for the purpose, and receive them back corrected on the following meeting night. Thus four exercises are mastered weekly-a by no means tardy pace. The signatures of the élèves are attached to the exercises, and the leader is thus enabled to keep a record of the mistakes, which he does, the object of which is to create a spirit of emulation among them. Taken as a whole, the French class is, in our opinion, conducted in a most satisfactory manner.

"In the Shorthand class we have upwards of a hundred members. It is, indeed, a most popular subject. In conducting this class we have experienced no difficulty whatever. It would be almost as easy to instruct a class of 500 as one of twenty members. The lessons in Phonographic Shorthand, as commenced in Part XIII., are explained and illustrated by means of a black-board; and having been so fortunate as to secure the services of a most proficient master, we are getting on very satisfactorily indeed.

"This is something of our present position. We are increasing very rapidly. The members thoroughly appreciate the value of the service rendered them; they are, in fact, enthusiastically in earnest. As soon as we can manage to meet twice instead of (as at present) once weekly, we purpose introducing other subjects, especially those of Latin, German, and Bookkeeping-for two of which subjects leaders have already volunteered. As a text-book we confine ourselves strictly to the POPULAR EDUCATOR, and experience justifies us in recommending similar classes in this particular, if in no other, carefully to follow our example. In no work that we have yet seen are the subjects treated on with such completeness and such admirable clarté, or so calculated to ensure the object of the learner-SUCCESS.

"In conclusion, allow me to add that it will be a matter of surprise to me if, in the course of a little time, POPULAR EDUCATOR CLASSES, on the principle laid down in your work, do not become truly national, both as regards their extent and character, and be established in all or nearly all the great towns and boroughs of the United Kingdom."

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WE saw in our last lesson that air, when heated, becomes sufciently rarefied to raise a balloon, together with its car and several passengers, but that there is great danger from fire. Now it was found that hydrogen gas was lighter than heated air, and hence it soon began to be used for inflating balloons, which were found to possess great lifting power. Pure hydrogen has less than th the weight of common air, 100 cubic inches of it weighing only about 2:14 grains. If, then, a balloon having a capacity of 16,000 cubic feet be filled with this, it will possess a lifting power of about half a ton. The hydrogen used was at first commonly made by the action of sulphuric acid and water on pieces of iron or zinc, the gas given off being passed through water to wash it from the acid, which would injure the balloon. It was, however, found to be expensive thus to make it, and being so light it soon mixed with the air, and thus lost much of its buoyancy; common coal gas is therefore now generally used, and if it be made at a somewhat higher temperature than usual it is sufficiently light for most purposes. Its specific gravity is much less, indeed, than that of pure hy. drogen, being about one-half that of the air, but increased size in the balloon will compensate for this, and it has the advantages of being cheap, easily procurable, and more manageable.

temperature of the air have also been taken, but no important practical results have as yet been achieved.

Having now seen proofs that the air has weight, we must see what effects this weight produces. If we lay a piece of iron or any substance on our hand it produces pressure, the amount of which varies with the weight of the body, and we should naturally expect the same effect to be produced by the air. A few simple experiments will show us that this is actually the case, and that the air does exert a very great pressure on every substance exposed to it. This pressure amounts to nearly 15 pounds on every square inch of surface. If we have a card measuring 4 inches by 3 inches, the pressure on it from the air will be 180 pounds. But it will be said this pressure is not felt, nor does the card bend at all; why is this? Simply because the pressure is equal in all directions, and therefore that on the lower side balances that on the upper. If we take away the air from the under side, we shall soon find that this is the case. This may easily be done by means of the airpump. We have only to procure a glass receiver, open at each end, and having stretched a piece of bladder or of thin india-rubber over one end, place it on the pump-plate, and exhaust the air (Fig. 3). The pressure above, not being balanced any longer by a corresponding pressure on the under side, will press the bladder down, and, after a few strokes, cause it to burst with a loud report. A thin piece of glass, if it be flat enough to close the top of the receiver air-tight, may be broken in a similar way, and thus will give a further proof of the intensity of the pressure. Instead of the bladder or glass the hand may be laid on the receiver, and will be pressed down forcibly as the air is removed. The cupping-glass, formerly so much used, acted on this principle. The air in it was rarefied either by means of a small syringe, or by heating the glass and allowing it to cool when placed over the required part of the body; the flesh was thus drawn up, and then pierced by the lancets.

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Fig. 3.

As the balloon ascends the pressure of the air becomes less and less, and thus the gas in it expands, and would soon burst open the silk, were it not that a large aperture is always left at the neck of the balloon to allow of the escape of any excess. So sensitive is this large body of gas to changes of pressure or temperature, that the fact of passing from a cloud into the rays of the sun makes a very perceptible change in its bulk.

The

A number of sand-bags are usually suspended outside the car, and when the aeronaut wishes to ascend he empties some of these, and thus diminishes the weight of the car and causes the balloon to shoot up rapidly. A large valve is also fixed at the top of the balloon, and can be opened by means of a cord which passes down into the car. When the tension becomes very great, or the aëronaut wishes to descend, he opens this, and thus allows a portion of the gas to escape. The greatest care is, however, necessary in descending, as much of the gas has escaped, and when the balloon nears the earth again it collapses to a considerable extent, and a further quantity of sand has to be let out. greatest elevation ever yet attained in a balloon was on the 5th of September, 1862, when Messrs. Coxwell and Gleisher made an ascent from Wolverhampton for the purpose of making some scientific experiments, and on this occasion a height of upwards of six miles was reached. Mr. Glaisher at this height became unconscious, and Mr. Coxwell had not strength enough left in his hand to open the valve, but was fortunately able to pull the string with his teeth, and thus to descend. At this time two-thirds of the atmosphere was beneath them, and the barometer stood at less than ten inches.

If a wooden cup, with a piece of cane let into the middle of it, be made to fit the top of the receiver and filled with mercury, the pressure of the air, when the receiver is exhausted, will drive the mercury through the pores of the cane, and it will fall like a

Fig. 4.

A grapnel is usually suspended from the car, and catching the earth gives assistance in descending; but with every care a violent shock will frequently be experienced, and the scientific instruments taken in the car be broken. A parachute is sometimes taken up with the balloon. This consists of a large circular piece of canvas with a small car suspended by ropes fixed at intervals round its edge. If the aeronaut gets into this, the canvas will open out like an umbrella, and the resistance of the air against it will be sufficient to break the force of the fall. At present no very great results have been obtained by the use of the balloon. In war one has occasionally been sent up fastened to a long rope, and from this the position of the enemy has been noted. Many valuable observations as to the state and

VOL. III.

fine shower into a vessel placed to receive it. Care must, however, be taken not to let any of it run into the pump, as it is almost certain to injure it seriously.

This pressure is exerted equally in all directions, and hence is almost unnoticed: a simple experiment, which all can try, affords à proof of this. Fill a wine-glass with water, so that it stands a little above the edge, and carefully slide a piece of card over the mouth so as to cover it completely and exclude the air; the whole may now be inverted without the card falling or the water being spilled. The sides of the wine-glass sustain the downward pressure of the air, and the upward pressure is more than sufficient to sustain the weight of the water in the glass. The only advantage of the card here is that it prevents the surface being broken up, and thus allowing the air to enter at places. A bottle with a very small mouth may be completely filled and carefully inverted without the water running out, even though the card be not used.

Several conjuring tricks are performed upon this principle, and many others are only ingenious applications of scientific principles, and appear startling mainly because these principles are so little understood by the majority of men. If a small hole be drilled in the bottom of a decanter, and the stopper put in firmly, no liquid will escape. The finger must, of course, be held over the opening while the liquid is being poured in. The decanter is then placed over a stand large enough to contain the wine, and on the stopper being slightly loosened the liquid will quietly flow out. A cover is placed over it while this is going In the same way liquids may be made to flow from small reservoirs concealed in covers into glasses placed upon them,

[graphic]

on.

74

and thus the wine appears to have passed from the decanter into the glasses. We see, thus, that there is science to be learnt even from some of the tricks of the conjuror, and a person who understands the principle of these things will be less likely to be deceived.

Perhaps the most striking proof of the pressure of the air is afforded by boiling some water in a thin cylinder provided with a stop-cock. After the water has been boiling for a little time all the air which was within is driven out, and it remains filled with steam. Let the stop-cock be now closed, and cold water poured over the cylinder; the steam inside will be instantaneously condensed into water, and the pressure of the air around will crumple up the cylinder, even though the cylindrical shape is best calculated to resist a pressure of this kind. If the steam inside a boiler be suddenly condensed by admitting cold water too rapidly, or in any other way, the boiler will sometimes collapse in a similar way. By ascertaining the area it exposes to the air, and multiplying the number of square inches in that by 14, we shall see how enormous the pressure thus exerted must be.

Another piece of apparatus, known as the Magdeburg hemispheres (Fig. 4), furnishes a very good illustration of this pressure. Two hemispheres usually made of brass are procured and their edges accurately ground, so that, when pressed together, they fit air-tight. A handle is fixed to each, that on one being so arranged as to screw on the end of a pipe closed by a tap and opening into the hemisphere. This handle is taken off, and the hemispheres pressed together, a little tallow being smeared on their edges; the pipe is then screwed into the hole in the centre of the pumpplate, and the air removed. When sufficiently exhausted the tap is closed, the apparatus unscrewed from the pump, and the handle replaced. Nearly all the air has now been removed from within them, and that around, therefore, presses them together with great force. If the hemispheres have a diameter of four or five inches, this pressure will be so great that it will be as much as two men can do to pull them apart. If, however, the tap be opened and the air re-admitted, the pressure inside will balance that outside, and they will fall apart by their own weight. It will easily be seen that, if their diameter be five inches, the portion of the pressure of the air which forces them together will be equal to that on a circular surface of the same diameter. Now, as the area of a circle is about 34 times the square of its radius, the area of this surface would be nearly 20 square inches, and we may assume that the air inside is so far rarefied that they are pressed together with a force of 14 pounds per square inch: the force required to separate them is therefore nearly 280 pounds.

A pair of hemispheres of this kind were constructed by Otto Guéricke, one of the inventors of the air-pump, of such a size that two teams of horses, pulling in opposite directions, were unable to separate them.

The pressure of the air furnishes us with explanations of many of the common phenomena of every-day life. When a cask is tapped, the beer soon ceases to flow unless a small hole be bored at the top, just as in the magic decanter the wine refused to run out of the small hole at the bottom until the air was admitted by the loosening of the stopper. A well-made cask is perfectly air-tight, and as soon as a little of the beer has been drawn from it the air inside becomes rarefied; the pressure of the external air is therefore greater, and, acting on the liquid in the tap, prevents its flow. As soon, however, as air is admitted by the spile-peg, the pressure is equalised, and the beer flows evenly. For the same reason a small hole is usually made in the lid of a teapot, so that when the water standing round the lid makes it nearly air-tight the tea may still flow in an even stream.

If we invert a wine-bottle, the liquid will flow out in a very irregular way; the air has to pass in by the neck at the same time as the contents flow out, and the meeting of the two produces the well-known gurgling sound. If, however, the bottle be slightly inclined, so that the air may enter at the upper part of the neck while the liquid flows out at the lower part, all this gurgling is avoided, and the liquid flows better and more rapidly.

We have another illustration of the pressure of the air in the common sucker, so well known to every boy. A piece of string, with a knot at the end of it, is passed through the centre of a circular piece of thick leather, which has been soaked in water till it has become quite soft and pliable. It is then pressed

closely down on a stone or flat surface so as to exclude the air from under it, and will be found to adhere so firmly that the stone may be raised by the string without its leaving the leather. The reason of this is that the moisture prevents the air entering between the stone and the leather. When, therefore, the leather is raised in the middle by the string a partial vacuum is produced, and the pressure which the air exerted on the upper surface of the stone is transferred to the leather and balanced by the tension of the string. The pressure of the air on the under side of the stone, being no longer balanced by a corresponding pressure above, lifts the stone.

It is frequently required to draw a small quantity of wine from a cask, as a sample, without tapping it: a small instrument known as a wine-taster is therefore made use of. This consists of a hollow tubular vessel, having a small aperture at each end, and somewhat bulged in the middle, as shown in Fig. 5. The bung is removed from the cask and the taster inserted. The wine soon rises through the opening at the lower end till it finds its level, the thumb is then placed over the opening above, and the air being thus prevented from entering, the wine is retained, and can be removed from the cask without any loss. On the thumb being removed from above air will enter, and the liquid will flow out into a glass.

Bird-cage fountains also depend for their action on the pressure of the air. The reservoir is constructed so as to be air-tight, and a small trough is placed near the lower end from which the birds may drink. This communicates with the reservoir by a small hole made at the level at which it is desired to maintain the Fig. 5. water. As soon as the water is so far removed from the trough that this hole is exposed, a bubble of air enters and displaces a small quantity of water, and in this way a uniform level is maintained and a constant supply of fresh water. Pneumatic inkstands have been constructed on a similar principle, the advantage resulting from their use being that a much less surface is exposed to the air, and therefore the ink does not thicken so rapidly, and also that there is less danger of spilling the contents.

We must now pass on to notice a very useful piece of apparatus the syphon.

In many manufacturing processes, and in chemical experiments, a liquid is often allowed to settle, and the clear liquor above has to be drawn off without disturbing the sediment, as would be done if the vessel were tilted so as to pour out the liquid. This can easily be accomplished by means of the syphon, which consists merely of a tube bent into the shape of the letter U, one limb, however, being longer than the other. In Fig. 6, c is the vessel from D which the water has to be drawn off, and C B represents the syphon. This is filled with water, and then, the ends being closed by the finger, inverted into the vessel; or it may be placed in the vessel, and the air exhausted from it by sucking with the mouth at the end B; the air being thus partly removed from the tube, the water rises and flows over the bend. The principle on which it acts is simply this: Let the syphon be full of water, and let us imagine a layer of water across the highest point of the bend to become solid. The forces which act on this layer and force it towards c are the pressure of the air acting upwards at B and the weight of a column of water equal in height to D C, for it is clear that this column has to be supported by the layer. The forces which drive it in the other direction, or towards B, are the pressure of the air on the surface of the liquid in c, which pressure is transmitted along the tube, and is just equal to the pressure at B, and the weight of a column of water equal in height to a B. It will thus be seen that the liquid will be moved along the syphon to

[graphic]

Fig. 6.

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