in number, the right and the left, and occupy the corresponding lateral cavities of the chest; they are conical in shape, the smaller end of the cone being placed the highest, and extending into the root of the neck from an inch to an inch and a half above the level of the first rib; the broad base of each lung rests upon the diaphragm, and extends lower behind than in front. Each lung is composed of two parts, called the upper and the lower lobes, which are separated from each other by a fissure. In the right lung the upper lobe is partly split into two by a shorter fissure, so that the right lung is said to have three lobes, whilst the left has only two. The right lung is always the largest; it is broader than the left, in consequence of the greater divergence of the heart to the left side; but this is in some measure compensated for by its being shorter, because of the liver forcing up the diaphragm to a higher level on the right side. About the centre of the inner surface of each lung is a spot where the bronchus, the pulmonary artery, and pulmonary vein, and nerves enter the substance of the lung; these structures, together with arteries and veins proper to the bronchus and the bronchial glands, are all enclosed in a process of the pleura, and form what is called the root of the lung. The two lungs taken together in the adult weigh from two pounds and three-quarters to three pounds; they are heavier in the male than the female. The colour of the lung varies with the age of the individual. At birth they are of a pinkish white; in the adult they become mottled with patches of a dark slate colour, in consequence of the deposit of colouring matter of a carbonaceous character; as old age advances, these patches become nearly black. The substance of the healthy lung is light and spongy, floating in water, and crackling when handled, a frothy fluid being squeezed out. In disease it often becomes solid, and is then heavier than water, and contains no air; this is one of the results of inflammation of the lungs.

We must now consider the minute structure of these curious organs. The substance proper of the lung is enclosed in a serous coat, derived from the pleura, and is made up of an infinite number of small divisions, called lobules, which, though closely bound together by connective tissue, are still quite distinct from each other. Each lobule is composed of a number of cells, called air-cells, clustered upon, and opening into, the terminal branches of the bronchi, with the minute divisions of the bloodvessels and nerves. When the bronchus enters the lung it divides into two, and these branches repeat the process until the ultimate ones have a diameter of less than the of an inch. In the largest branches the structure remains the same as in the bronchus; they have walls, formed of tough membrane and imperfect cartilaginous rings, by which they are held open; but as they attain their greater degree of minuteness, the walls consist simply of membrane. Into these smaller ones the aircells open, and over them the pulmonary capillaries spread their close network. The air-cells vary much in form, according to the amount of pressure to which they are subjected; their walls, which are nearly in contact, are formed of very thin membrane. The size of an air-cell is from the to the of an inch in diameter; they communicate freely with each other, and are, as before stated, arranged in groups along the sides of the bronchial tubes. It has been estimated that the total number in the lungs exceeds 600,000,000. Outside of these cells and tubes the capillary plexus is so dense, that the meshes are narrower than the vessels which compose them; the capillaries here have an average diameter of the of an inch. Thus the blood is brought into the most intimate relation with the air contained in these myriads of cells, there being nothing interposed between them but the very thin walls of the cells and capillaries, and frequently, this bringing the blood and air together, is even more perfectly provided for, as one capillary will often have a layer of air-cells on each side of it. The cells of one lobule do not communicate with those of another, and consequently if the bronchial tube going to a lobule become stopped, the supply of air to that lobule ceases, and it is rendered useless.

The function of respiration consists of two distinct acts, called respectively inspiration (by which the lungs are inflated with air) and expiration (by which the air, after having served its purpose, is driven out of the lungs). To understand this process, we must fix firmly in our minds the conditions under which it is performed. The highly elastic lungs are enclosed in the cavity of the thorax, the bony framework of which is com

pleted in all its deficient parts by muscular structure, and the capacity of which is capable of great alteration by muscular agency. Likewise we must remember that in the healthy living body no such thing as the cavity of the thorax exists; the lungs and heart completely fill up this space, and are in close relation to its walls in every part. The result of these arrangements is that when by any means the capacity of the chest is diminished, air is driven out of the lungs, and when the pressure is removed the lungs by their elasticity expand and follow the walls of the thorax, and so create a vacuum in some of the air-cells, and the atmospheric air at once rushes in through the windpipe to fill the empty cells. During inspiration, the capacity of the chest is, as a rule, increased in every direction, but the way in which this increase is obtained varies in different instances. In young children the act of inspiration is performed almost entirely by the diaphragm, which, descending, forces down the contents of the abdomen, and so increases the size of the chest. In the adult, in addition to the diaphragm, which still performs a large part of the work, the elevation of the ribs by the numerous muscles attached to them comes into play; and in consequence of the way in which the ribs are articulated with the spine, and their cartilages with the breastbone, making the centre of the rib the lowest point, any raising of the ribs at the same time draws them outwards, and the ends being both more or less fixed tends to bring the ribs into nearly a straight line with the cartilages, and so, as a matter of course, enlarges in a very marked degree the capacity of the chest. This action will be at once understood, if reference be made to the illustration of the thorax, given in one of the earlier lessons on this subject. The chest and lungs during expiration resume their ordinary size by reason of their elasticity, which in deep expiration is aided by the abdominal muscles contracting and forcing up the diaphragm, which remains passive during expiration.

The quantity of air changed at each inspiration varies in different people, and this variation has been taken as a measure or index of the physical strength and constitution of the individual. Thus it has been found by experiment that a healthy man five feet seven inches in height can expire 225 cubic inches of air, and that for every additional inch of stature an increase of eight cubic inches in the capacity takes place. This rule is not much affected by the weight of the person, but age is found to modify it to a certain extent; thus the capacity increases from about the fifteenth to the thirty-fifth year, and then gradually diminishes. The number of respirations in the minute is, on an average, from fourteen to eighteen in a state of repose of body and mind; but this is liable to great variation from disease, mental emotion, or physical exertion.

The purpose of this function of respiration is to submit the blood charged with the waste material of the body to the purifying action of the air; from this contact of the blood with the air, certain changes are induced in both the blood and the air; these must now be examined, and, as a preliminary, we must stop for a minute, and see of what the atmospheric air is composed. In almost all positions the composition of the air is identical, and for our present purpose it will be enough to say that it contains oxygen, nitrogen, carbonic acid, and watery vapour. There is in it about 21 per cent. of oxygen to 79 per cent. of nitrogen by measure, or 23 per cent. of oxygen to 77 per cent. of nitrogen by weight. The quantity of carbonic acid is very small, not more than 4 to 5 parts in 10,000. The quantity of water in a state of vapour varies greatly, being influenced by temperature and other causes; but it is never entirely absent from the atmosphere. The changes which take place in the air during respiration are as follow: First, the oxygen is diminished; secondly, the carbonic acid is increased; thirdly, the temperature is raised; fourthly, the moisture is increased. Of these changes the first two are by far the most important, and may be considered together, as one is in a great measure dependent on the other. The oxygen is diminished, because it is absorbed, and enters into combination with the surplus carbon of the system, to form carbonic acid-not that the whole of the oxygen absorbed is utilised in this manner; some of it, doubtless, assists in forming some of the other compounds carried out of the body by means of the skin and kidneys. The quantity of oxygen absorbed varies with different circumstances and in different individuals. Animals of a small size consume a much larger quantity in proportion to their size than larger ones. The kind of food on which an

animal lives also influences the consumption; it is considerably greater on an animal than on a farinaceous diet.

The increase of the carbonic acid is mainly dependent on the absorption of oxygen, and this, therefore, is also affected by like circumstances. In an ordinary way, it is calculated that a man exhales 173 grains of carbon per hour, or rather more than eight ounces in the twenty-four hours. Other authorities place it as low as five, whilst Liebig estimates the amount from the skin and lungs together at nearly fourteen ounces. Age and sex have some influence in this matter: thus, the amount in males regularly increases from eight to thirty years of age, and from forty to extreme old age steadily diminishes. Temperature also affects the result, the higher the temperature the less the amount of carbonic acid exhaled. If the air is impure, as where there is not sufficient ventilation, and in consequence the same air is breathed more than once, the quantity of carbonic acid is diminished, showing that in the absence of the proper proportion of oxygen, the necessary purification of the blood does not take place. By food the quantity is increased, by fasting it is diminished; physical exertion increases it; rest, especially sleep, diminishes it. The temperature of the expired air is in ahnost all cases raised, its average heat being about 98 to 99 degrees. The moisture is also always increased, the increase being greater the dryer the air is before it is inspired, the expired air being always nearly saturated with moisture. The quantity of water given off by the lungs during the twentyfour hours is estimated to vary from six to twenty-seven

ounces.

The changes produced in the blood during respiration are manifested, first, by change of colour-the dark venous blood acquiring the bright arterial hue during its passage through the lungs; secondly, by the temperature of the blood being raised by the same process. The way in which the oxygen inspired is absorbed, and the carbonic acid expired is formed, has been much disputed. It used to be formerly held that the oxygen at once, at its entrance into the lungs, combined with the carbon contained in the blood, and thus formed the carbonic acid; but it has now been conclusively shown that though, no doubt, some of the carbonic acid is produced in this way, yet the greater part exists already in the blood by the time it reaches the lungs. The origin of this, the larger part of the carbonic acid, is thus explained :-When the venous blood is passing through the lungs, it gives up the carbonic acid with which it is charged, and absorbs the oxygen, the red corpuscles being credited with the greater part of this work; the oxygen thus held in solution, and not in combination, by the aërated blood, is conveyed by the arteries to the capillary system, where it is brought into intimate relation with the elementary tissues, and the oxygen assists in the nutrition of the system, and, combin

ing with the waste carbon of the worn-out structures, forms carbonic acid and water, which are conveyed by the veins back to the lungs, there to be removed from the body. In their office of purification the lungs are powerfully assisted by the skin. From the whole surface of the body there is constantly going on an exudation of watery fluid containing many elements derived from the wasted tissue-and, notably, carbonic acid. It is very difficult to estimate the amount of carbonic acid thus excreted, but the importance of the proper performance of this function of the skin is proved by the fact that animals whose skin had been covered with an impermeable varnish, and thus prevented from doing its duty, soon died with all the symptoms of suffocation. This also shows how necessary for the preservation of health it is that the skin should be kept healthy and active by the free use of baths, etc., to clear away the exuded material from its surface.

Closely connected with this function of respiration is the question of animal heat, and the causes which produce and maintain it; the chief, if not the only one, is that combination of oxygen with the various other elements of the body, which is mainly brought about by respiration. The formation of carbonic acid, water, etc., in the body are all instances of chemical action, and heat is necessarily produced; and as these changes are continually going on in all parts of the body, it follows that a greater or less amount of increase of temperature is also being constantly brought about. And it has been conclusively shown by experiment that sufficient, or nearly sufficient, heat is produced during these processes to account for all the animal heat of the body.

LESSONS IN ALGEBRA.-XL. CONTINUED GEOMETRICAL PROPORTION OR PROGRESSION, WHEN all the ratios of a series of proportionals are equal, the quantities are said to be in continued proportion or progression, As arithmetical proportion continued is arithmetical progression, so geometrical proportion continued is geometrical progression. It is sometimes called progression by quotient. The numbers 64, 32, 16, 8, 4, are in continued geometrical proportion.

In this series, if each preceding term be divided by the common ratio, the quotient will be the following term. Thus, 2 = 32, and 32 16, and 8, and ¦ = 4.

=

This common multiplier or divisor is called the ratio. For ratio as always a multiplier, either integral or fractional. most purposes, however, it will be more simple to consider the

In the series 64, 32, 16, 8, 4, the ratio is either 2 considered as a divisor, or considered as a multiplier. When several quantities are in continued proportion, the number of couplets, and of course the number of ratios, is one less than the number of quantities. Thus the five proportional ratios; and the ratio of a: e is equal to the ratio of a:b, tut quantities, a, b, c, d, e, form four couplets containing four is, the ratio of the fourth power of the first quantity to the fourth power of the second. Hence,

the square of the first to the square of the second, or as the If three quantities are proportional, the first is to the third as square of the second to the square of the third. In other words, the first has to the third a duplicate ratio of the first to the second. And conversely, if the first of the three quantities is to the third as the square of the first to the square of the second, the three quantities are proportional.

If a bbc, then a: c:: a: b2. And universally,

the first to the last is equal to one of the intervening ratios If several quantities are in continued proportion, the ratio of raised to a power whose index is one less than the number of quantities.

If there are four proportionals, a, b, c, d, then a:d::a3: . If there are five, a, b, c, d, e; a: e:: a*: b', etc.

If several quantities are in continued proportion, they will be proportional when the order of the whole is inverted. This has already been proved with respect to four proportional quantities. It may be extended to any number of quantities. Between the numbers, 64, 32, 16, 8, 4, 2, 2, 2, 2.

The ratios are,

Between the same inverted, The ratios are,

4, 8, 16, 32, 64, 1, 1, 1, 1.

So if the order of any proportional quantities be inverted, the ratios in one series will be the reciprocals of those in the other. For by the inversion each antecedent becomes a consequent, and vice versâ; but the ratio of a consequent to its antecedent is the reciprocal of the ratio of the antecedent to the consequent. That the reciprocals of equal quantities are themselves equal is evident from Ax. 4.

To investigate the properties of geometrical progression, we may take nearly the same course as in arithmetical progression, observing to substitute continual multiplication and division instead of addition and subtraction. It is evident, in the first place, that,

In an ascending geometrical series, each succeeding tern is found by multiplying the ratio into the preceding term. If the first term is a, and the ratio r, Then a xar, the second term; ar X rar, the third; ar2 xr = = ar", the fourth; ar3 × r = ar1, the fifth, etc. And the series is a, ar, ar2, ar3, art, ar3, etc.

If the first term and the ratio are the same, the progression is simply a series of powers.

If the first term and ratio are each equal to r, Then rr2, the second term; r2 xr =3, the third; prẻ xr =3, the fifth.

, the fourth;

1

+1=r, the ratio.

When the ratio is found, the means are obtained by continued multiplication.

The next thing to be attended to is the rule for finding the sum of all the terms.

If any term, in a geometrical series, be multiplied by the ratio, the product will be the succeeding term. Of course, if each of the terms be multiplied by the ratio, a new series will be produced, in which all the terms except the last will be the same, as all except the first in the other series. To make this plain, let the new series be written under the other, in such a

manner that each term shall be removed one step to the right of that from which it is produced in the line above. Take, for instance, the series, 2, 4, 8, 16, 32. Multiplying each term by the ratio, 4, 8, 16, 32, 64. Here it will be seen at once that the last four terms in the upper line are the same as the first four in the lower line. The only terms which are not in both, are the first of the one series, and the last of the other. So that when we subtract the one series from the other, all the terms except these two will disappear, by balancing each other.

To find the sum of a geometrical series:

Multiply the last term into the ratio, from the product subtract the first term, and divide the remainder by the ratio less one. EXAMPLE. If in a series of numbers in geometrical progression, the first term is 6, the last term 1458, and the ratio 3, what is the sum of all the terms?

1. Find two geometrical means between 4 and 256

2. Find three geometrical means between and 9.

3. If the first term of a decreasing geometrical series is, the ratio

, and the number of terms 5, what is the sum of the series ?

4. What is the sum of the series 1, 3, 9, 27, etc., to 7 terms?

5. What is the sum of ten terms of the series 1, 3, 3, 5, etc. ?

6. If the first term of a series is 2, the ratio 2, and the number of terms 13, what is the last term ?

7. What is the 12th term of a series, the first term of which is 3, and the ratio 3 ? Also find the sum of the series. 8. A man bought a horse, agreeing to give one farthing for the first nail in his shoes, three for the second, and so on. The shoes contained 32 nails; what was the cost of the horse?

Quantities in geometrical progression are proportional to their differences.

Let the series be a, ar, ar2, ar3, ar1, etc.

By the nature of geometrical progression,

a: ar:: ar: ar2 : : ar2: ar3: : ar3: art, etc.

In each couplet let the antecedent be subtracted from the consequent.

Then a ar:: ar a: ar ar :: ar ar: ar3 - ar2, etc. That is, the first term is to the second, as the difference between the first and second to the difference between the second and third; and as the difference between the second and third to the difference between the third and fourth, etc. If quantities are in geometrical progression, their differences are also in geometrical progression. Thus the numbers

3, 9, 27, 81, 243, etc. 6, 18, 54, 162, etc.,

And their differences are in geometrical progression.

Problems in geometrical progression may be solved, as in other parts of algebra, by means of equations.

EXAMPLE.-Find three numbers in geometrical progression, such that their sum shall be 14, and the sum of their squares 84. Let the three numbers be x, y, and z. XY:: Yz, or xz= = y2

By the conditions,

And

And

x+y+z=14. x2 + y2 + x2 = 84.

From these three equations, x, y, and 2= 2, 4 and 8. Ans.

EXERCISE 72.

duct is 64, and the sum of their cubes is 584. What are the numbers? 2. There are three numbers in geometrical progression: the sum of the first and last is 52, and the square of the mean is 100. What are the numbers ?

1. There are three numbers in geometrical progression whose pro

3. Of four numbers in geometrical progression, the sum of the first two is 15, and the sum of the last two is 60. What are the numbers ? 4. A gentleman divided £210 among three servants, in such a manner that their portions were in geometrical progression; and the first had £90 more than the last, How much had each ?

5. There are three numbers in geometrical progression, the greatest of which exceeds the least by 15; and the difference of the squares of the greatest and the least is to the sum of the squares of all the three numbers as 5 to 7. What are the numbers ?

6. There are four numbers in geometrical progression, the second of which is less than the fourth by 21; and the sum of the extremes is to the sum of the means as 7 to 3. What are the numbers?

KEY TO EXERCISES IN LESSONS IN ALGEBRA.-XXXIX.

1. 3.

2. 24 and 32.

3. 10 and 8.

4. 8 and 6.

9. 15 and 9.

13. 24 and 16.

14. 20 and 16.

RECREATIVE SCIENCE.-XXI.

THE PHENAKISTISCOPE-BEALE'S AUTOMATIC FACE-THE WHEEL ANIMALCULE.

FORTY years ago an amusing toy was invented, called the Phenakistiscope, from the Greek pevakiw, to deceive. It consisted of a number of devices painted round the circumference of a disc, each device similar in its general subject, but having a difference of position in some of its details in each successive compartment; in short, a complete action, as that of a ball running around the interior of a ring, was finished in twelve repeats of the device.

The method of observing the singular illusion of the phenakistiscope, until Mr. Rose's kalotrope was invented, was to look upon its reflection in a mirror, through slits or openings in the disc, whilst it was in rapid revolution. This mode of using the disc of course confined its use for the time to one individual; and notwithstanding repeated attempts by the optician, no movement had been contrived by which the illusion could be shown to an entire audience.

Since the invention of this interesting toy, various eminent opticians sought earnestly for the means of showing it to a whole company at once, and this with a view to making it a powerful and interesting aid in the production of effects to be exhibited by the magic lantern. All attempts before the invention of Mr. Rose's kalotrope, failed, either from want of efficiency, or from their being too intricate and expensive, or from both of these causes together.

An apparatus for the magic lantern, constructed by Messrs. Duboscq, answered pretty fairly, but was very limited in its results, and this was quite superseded by the larger and more scientific contrivance invented and given to the writer by Mr. Rose, called the Photodrome (light-runner, or lightcourse), already alluded to at page 233 of this volume of the POPULAR EDUCATOR.

H

G

turned by the handle, c. A wheel, D, keyed on to the shaft, B, drives a pinion, E, say in the ratio of two to one; the pinion carries round a shaft, having fixed on it a slotted disc, F, perforated with eight radial slots or apertures. When a powerful artificial light, either the oxy-hydrogen, or lime-light, or electric light, is arranged in a box with a condensing lens, so as to throw the point of the cone of rays, or focus of rays, through one of the apertures, and of course, when the slotted dise rotates, eight flashes of light will pass at each revolution, and as the wheel and pinion are two to one, sixteen flashes of light will pass, and illuminate the screen G when the automatic face is shown for each turn of the handle c.

Let us now suppose that on the screen & the smart hat and feathers, curls, neck, shoulders, and bust of a young lady are depicted, but that, instead of a face being shown, an opening is cut through the screen G of the size of the human face, and that on the circular disc,or card, H (Fig.2), which is carried round by the shaft B, and is behind though close up to the screen G, sixteen faces are so arranged that one of these faces shall be in the right position to register with the hole cut through the screen G at each flash of light, so that the face and the hat, curls, neck, shoulders, bust, make up together one complete picture of the human face divine. It is evident that if the six

B

D

A further interest was imparted to this class of optical deceptions by the construction of the Zoetrope, or Wheel of Life, by which a large family party may be amused by the curious movements of various figures seen through slits in a revolving circular box, in which the slips of paper bearing the devices are placed, and changed at pleasure. The enormous sale of these toys reminds the historian of optical toys that the like success attended the sale of Sir David Brewster's kaleidoscope; indeed, the popularity of both contrivances brings home to us the truth of Goldsmith's words

"And still they gazed, and still the wonder grew."

And deservedly so, for both have become regular inmates of the toy cupboard. There are, however optical contrivances which take a high position on account of the very ingenious manner in which they are contrived; and amongst machines that illustrate the various phases of "persistence of vision," none are more interesting than those invented by Mr. John Beale, of Greenwich-invented not for any personal advantage, but for the

advancement of scientific recreation.

The first to be described is the "Automatic Picture," in which the face of a charming young lady, waking from an apparent lethargy, rolls its eyes, opens and shuts its mouth, and occasionally, for the special delectation of "rude" boys, pops out its tongue, or varies the amusement by grinning "horribly a ghastly smile," very provocative of merriment, and useful, as Dr. Walcot says, for

teen faces were all painted alike, the resultant picture would, when the discs were revolving and the flashes of brilliant light illuminating the screen G, be apparently the same as when the apparatus was at rest, i.e., when any one of the sixteen drawings of the faces is flush with the opening cut through the screen G. If, however, the sixteen faces are all painted with different expressions, viz., one of them with the eyes and mouth closed, the next with the eyes and mouth a trifle open, and so on with each succeeding picture of the series, opening the eyes and mouth more and more, then an appearance of opening the eyes and month would be given, but, like all other illusions of this class, & mere repetition of the same effects in fixed order would be produced. Mr. Beale's automatic face apparatus enables the experimentalist to produce a novel and curious spontaneous movement on the part of the lady,", who elegantly diversifies the effects already mentioned by thrusting out her tongue occasionally.

66

This part of the illusory effect is thus obtained: 'the sixteen faces form two distinct groups of eight each, as arranged and painted on the card disc, Fig. 2; the figures 1 to 8 are faces arranged as already described, showing by a series of gradations the opening and shutting of the eyes and mouth; and the letters rolling from side to side, and instead of displaying the opening a toh are the other group, showing eyes always open, but and shutting of the mouth, suppose the tongue to be gradually protruded so as to hang in a négligé style over the chin, it is evident that the operator has two groups of face pictures painted on the same disc, making different motions. It must be remembered that the sixteen faces are successively illuminated by flashes of light through the eight slots, or apertures, and it will be seen that if every other slot is closed, so as the sixteen faces; and if those slots are closed in their turn, to leave four apertures open, they will illuminate one group of the other four apertures will illuminate the second group of the sixteen faces.

"Care to our coffin adds a nail, no doubt;

And every grin, so merry, draws one out."

with the surface of the slotted or perforated disc. It must necessarily occur that in the course of movements the interceptor allows sometimes one group of painted faces, and sometimes another, to be seen; but shifting its position at varying and uncertain intervals of time, it produces an effect which gives the spectator the idea that the lady is truly "automatic," and can do as she pleases.

The interceptor is shown at Fig. 3, with the face in front of it, and the curtains should always be arranged so as to hide the automatic lady until the shaft B (Fig. 1) is in motion, and all light, except that passing through the slots from the oxy-hydrogen lantern, carefully excluded. Of course other figures may be used, but there are none better than the one suggested, because refinement should be a matter of course with the fair sex; and it seems to be such a libel on a pretty face to convict it of a want of good breeding, and to verify part of Doctor Johnson's epitaph for Hogarth

"The attentive eyes,

That saw the manner in the face."

ん

These parts are never seen as wheels, except in motion; the animal is sometimes seen without them, the parts which produce the appearance being then either retracted and drawn inwards, or disposed in other forms, for the animal is of a very changeable nature. The motion of the wheels is continuous, as if they were spinning constantly in one direction upon their axis; the velocity is such as to carry the teeth rapidly before the eye, but is not enough to confound the impression of one tooth with that of its neighbour, and therefore they may be distinctly seen.

2

Ъ

H

The spectator will see the tongue sometimes thrust out, and the eyes wildly rolling for a dozen times in succession; then perhaps the eyes suddenly close or open, and the mouth gapes wide open; and then, in the middle of an elegant yawn, the mouth is suddenly closed, and the tongue protruded, because the interceptor changes its position by variations in the friction of the perforated wheel, and always at uncertain and varying intervals. To recapitulate, Fig. 1 shows the whole apparatus in section; Fig. 3, the back of the slotted disc and the interceptor, the lantern being removed to allow the latter to be well shown. The lantern throws the light through the slots to the face, which is concealed by the curtains until the movements are commenced and the room darkened. Fig. 2 is the cardboard disc, upon which the sixteen faces are painted in groups, as already described; and this works behind the screen upon which the hat, curls, neck, shoulders, etc., of the "automatic lady" are depicted.

By artificial means the semblance of nature is given to a picture of the human face; but even here it cannot be said that the idea is new, because Nature herself appears to simulate motions, some of which attracted the attention of Faraday, and formed part of the subject of a very interesting paper written by him in the Journal of the Royal Institution, vol. i., p. 220. The paper refers to the curious appearance exhibited by the "Wheel Animalcule."

This little insect has been well described by Mr. Baker and others, and can only be viewed distinctly under a high magnifying power; it then presents an elongated, sack-like form, either attached by the posterior part to the side of the vessel

G

containing the water in which it exists, or else floating in the fluid. When the effect in question is observable, there is seen the appearance of two wheels, one on each side of the head; they seem formed of deep teeth or short radii, perhaps fourteen or fifteen in number. The form of these teeth is not sharp or well defined, but hazy at the edges; the interval between them is perhaps rather more than the width of the teeth. The teeth are not distinctly set on to a nave or axis, but appear sometimes even to melt away, or attenuate, at the part towards the centre, and sometimes appear as independent portions, i.c., as much separated from the centre part, or supposed place of attachment, as from the neighbouring teeth.

R

In this and a former lesson the toy called the Zoetrope, or Wheel of Life, has been mentioned, and, as it is a simple contrivance, and easy of construction, we may as well give a detailed description of it here, so that any of our readers who are mechanically inclined may be able to make one for themselves.

ness.

First it is necessary to procure a circular disc of wood, ten inches in diameter and half an inch or rather more in thickThis disc should be made to revolve freely on an iron pin put into a handle like the handle of a skipping-rope or bradawl, which must have a disc of wood about three inches in diameter at the top, on which the larger disc may rest, and which will prevent it from having an oscillating motion from side to side when it is turned rapidly round. The larger disc should be prevented from slipping off the pin on which it revolves by means of a small nut. A long screw passed through the larger disc into the handle will do as well as the iron pin and nut.

Next take a strip of pasteboard thirty-three inches in length and seven in breadth. This will be enough to go round the disc and leave one and a half inches for lapping over and joining up the pasteboard into a cylindrical form, which may be

effected by a few stitches, or by means of brass rivets similar to those generally used by shoemakers and staymakers. Having done this, divide the pasteboard into two equal parts by a line drawn along its length, and in the upper part cut eleven or thirteen vertical slits, two and three-quarters inches long and three-sixteenths of an inch broad, at equal distances from one another, the bottom of each slit touching the line drawn across the pasteboard. Care must be taken to cut the slits in such a manner that there may be the same distance between the slits at each end when the cylinder is joined up as there is between any two of the slits; and before cutting them out it will be as well to paste white paper on the side of the pasteboard intended for the inside of the cylinder and black paper on the outside. The slits should then be cut out with a sharp penknife. When the cylinder has been joined up so as to fit exactly round the disc, it may be fastened to it by means of a few iron tacks, taking care that the perforated part remains uppermost.

Fig. 3.

Strips of paper, three inches wide and long enough to fit round the interior of the cylinder and lap over a little, should now be taken, and figures, similar in form and colour, but in different attitudes, drawn on them. The figures should be equal in number to the slits in the cylinder, and so drawn that when the strip is placed within it a figure may appear exactly under each slit. When the above directions have been attended to, and the cylinder is caused to revolve by the hand, the eye being at the same time directed to the interior through the slits, the figures will appear to be endowed with motion.