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arms are shorter, more approaching the proportions of the plicable to the apes in general. All the species of this family human; the thumb is more developed, and the throat is not so have opposable thumbs on the four hands, and can therefore nga much disfigured by the singular sac, which swells when the orang both thumbs and fingers in grasping. Man has this structure utters its loud and fierce cry.

in the hands

only, the great toe being wholly useless for such a The chimpanzee is a native of Western Africa, and was long purpose. The case is reversed in many of the American supposed to be the only ape of

monkeys, which have the opthis continent, the magot or

posable thumb on the hind. Barbary ape (Simia Innuus),

limbs only, and are therefore as it is often called, being

sometimes called pedimana more allied to the baboons.

(hand-like feet). A glance at Two other species—at least,

the hand of the chimpanzee the gorilla and kooloo-kamba

or the gorilla will show the -have, however, recently been

vast superiority of the human discovered in Equatorial Africa,

thumb and hand over that which seems likely to gain the

possessed by any ape. honour of being deemed the

While the feet or hinder special land of the apes. Some

limbs of these quadrumans are years ago, three chimpanzees

fitted for grasping, their strucmight have been seen at the

ture renders it impossible to same time in the Regent's

plant the sole of the foot Park Gardens; but a year has

firmly on the ground. The passed since the death of the

outer edge only rests on the last specimen, and its place is

earth, and hence the rolling still vacant.

motion of these animals when THE GORILLA. — This re

moving on the ground. The markable ape of Equatorial

mode in which the foot is Africa has only recently been

joined to the ankle inevitably made known to Europeans.

forces the sole to take 21 It is true that the Carthagi. HEAD OF YOUNG CHIMPANZEE.

oblique and inward direction, nian navigator, Hanno, was

admirably qualifying the ape supposed to have taken the

to climb, but not to walk. skins of three to Carthago; but these animals are now thought The comparative length of the arms in man and the various to have been chimpanzees, though called gorillas by the Greeks, species of apes deserves attention. The human hand reaches to who translated the account of Hanno's wonderful voyage. To the middle of the thigh ; that of the gorilla to the knee. The M. Paul du Chaillu belongs the honour of describing the gorilla chimpanzee's fingers extend a little lower; those of the orang in its forest home, and the "Memoir” of Professor Owen gives touch the ankle, and in some of the gibbons extend to the a full account of the animal's

ground. The following comstructure. No living specimens

parison between the capacity of these apes have reached

of the human and ape skulls England, but a good notion of

yields some instructive retheir forms and enormous

sults. The average capacity strength may be gained from

of an English skull is 96 cubic the admirably set-up indivi.

inches; of a Hottentot's, 75; dual killed at the Gaboon river,

the gorilla's, 344; the chimand now to be seen in the

panzee's, 27; and that of the British Museum.

orang, 26 inches. The first opportunity for ex

How many changes does the amining any part of a gorilla

reader suppose would be newas given in the year 1847,

cessary to make the body of a when a cranium was brought

gorilla like that of a man! to England, and shortly after.

Twenty-four alterations of wards two other skulls were

structure at least would be readded. But in 1858 the body

quired to effect this transmu. of a gorilla, preserved in a

tation in the physical organicask of spirits, arrived at the

sation only. Thus it will be British Museum, and

seen that numerous differences treated with all the anatomical

separate even the bodies of honours due to such a visitor.

the apes from those of men. The skin, bones, brain, nerves,

We shall not here attempt to and muscles were carefully

explore the width and depth examined by Professor Owen,

of the gulf which separates the and photographs of the animal,

intelligence of these creatures in various positions, were

from ours.

Some have vere taken. All these representa

tured to inquire why those tions may be studied in the

animals do not speak, as their “Memoir on the Gorilla."

organs of speech resemble This ape bears a bad name

HAND OF CHIMPANZEE. abroad, and is thoroughly

FOOT OF CHIMPANZEE.

man's? These inquirers for

get that organs of speech must dreaded by the negroes, and

act according to the power of especially by the women, who firmly believe that these animals the mind which employs them ; hence while man uses a glottis delight to carry them off to their forest haunts. The hideous and vocal chords to form a language, the apes can but employ roar uttered by the gorilla when he advances to the attack, the the same organs to produce a bark or a yell. ferocious courage of the creature, and the death-stroke which Many other topics are suggested respecting a family of ani. tears open the body of the assailant,

have made this quadruman mals so mysteriously resembling, and yet so widely differing the terror of the forests. Superstition aids this feeling, as the from man; but we must refrain from entering upon these ques. natives think the spirits of certain deceased men enter into some tions. The preceding observations may incite some readers to gorillas, and endow them with human cunning and energies. search more minutely into the numerous and important diffe

We will now conclude this paper with a few remarks ap-rences between the lowest man and the highest quadrumana,

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was

PNEUMATICS.–VII.

imperceptible. Higher still the current is reversed, and we shall VENTILATION-WIND—ITS PRESSURE AND EFFECTS-TRADE escaping near the upper part of the room.

find the flame driven outwards, showing that the heated air is WINDS—BONSOONS-LAND AND SEA BREEZES-SIMOOMS, ETC.

This experiment is a very instructive one, as it explains to us A SIMPLE experiment, which may be easily tried, affords a good the principle on which rooms must be warmed, and generally the illustration of the principles on which ventilation is arranged in way in which gases become heated. If we place one end of a mines or buildings. Procure a straight glass chimney (Fig. 20), bar of iron in the fire, the heat will gradually pass along it until such as is used with a moderator lamp, and place it over a short the other end becomes warm. The particles do not change their

piece of lighted candlo; in a few minutes the position with regard to one another, but the heat passes from
flame will flicker and then be extinguished. each to that next to it, and thus is conducted along the bar.
The oxygen of the air contained in the Liquids and gases, however, become heated in a totally
chimney has been consumed, and its place different way. Instead of the heat being conveyed from one
occupied by the poisonous carbonic acid gas particle to another by contact, currents are set up in them by
thereby produced ; and as there is no way which all the particles successively become exposed to the source
of allowing the foul air to escape and fresh of heat. These currents may easily be seen by dropping some
to enter, the candle goes out. Now cut a fragments of litmus into a globe of water, and heating it by
piece of cardboard of the shape represented means of a spirit-lamp placed under the centre. The litmus
at A, such that it will pass down the chim. will colour the water, and thus it will be seen that there is an
ney nearly to the flame, but will be pre- ascending column of liquid in the centre of the globe. When
vented by the enlarged top from falling this reaches the surface it spreads out in all directions, and
lower, and, having re-lit the candle, place the descends against the sides, to be raised to a higher temperature
chimney over it as before, but with the card and again ascend. We may easily see how bad a conductor of
in it. The candle will now continue to burn, heat a liquid is. If we place a lump of ice at the bottom of a
and we shall find that the flame is drawn test-tube nearly filled with water, and apply the flame of a
a little to one side. The partition has di- spirit-lamp to the upper portion, we shall find that the ice will
vided the chimney into two compartments, remain unmelted long after the water at the top has commenced
up one of which the smoke and carbonic to boil. If, however, we apply the flame to the bottom of the

acid pass, while fresh air comes down the tube, the ice will very quickly melt. In a similar way it might Fig. 20.

other and maintains a constant supply. The be shown that air is a bad conductor, and can therefore only be

heat of the flame rarefies the air over it, and warmed by convection. thus creates an upward current in the one compartment, just as Another important fact in connection with the temperature of the furnace does at the bottom of a mine shaft.

the air is, that it is not warmed by the passage of the sun's rays If, instead of inserting the piece of card, we raise the chimnoy through it. If it were, the upper part of the atmosphere would a little from the table by a few small pieces of wood, the candle in the daytime be warmer than the lower portion, for it is nearer will continue to burn ; an inlet is now provided for the fresh the sun; we find, however, that as we ascend the temperature sir, and the whole of the chimney is occupied by the up-current. becomes less and loss. An interesting illustration of this is The essential points are to provide an exit for the foul air and seen on the higher tropical mountains. If we go to some of the an entry for fresh air.

lofty Andes near the equator, we shall find at their base tropical In a building the air, heated by the lights and by being forests almost ton dense for the light of the sun to penetrate ; breathed, ascends. This will usually suffice to ventilate the on ascending higher we meet with vegetation less rank, and pass place if a proper way of escape be made for the foul air, and the through regions resembling those of the temperate zone. Higher importance of doing this cannot well be overrated. Care, how still we come across a stunted polar vegetation, and reach the ever, is required in regulating the entry of fresh air so as to limit of perpetual frost, or the "snow-line. We have, in fact, avoid draughts.

in a small space a representation of all the climates we meet in Frequently, however, the gas is not lighted, and the heat of travelling from the equator to the poles. The reason of this is the breath is insufficient to ventilate the building in this way, that the rays of the sun pass through the air without warming and then other means must be adopted. One of the simplest it, but when they reach the ground they are absorbed and raise and best of these is to carry a metal shaft from the top of the its temperature. This increase is shared by the air resting on room or building, and in the upper part of this to place a number it, and thus the lower strata become warmed. of gas-burners. These, when lighted, create a powerful current As we recede from the equator, the snow-line, of air up the shaft, and thus the foul air is removed from the which there attains an elevation of 17,000 or place.

18,000 feet, descends lower and lower, till in Gas-burners have also been invented of such a construction as the frigid zone it comes to the level of the sea, to remove the impure air by means of a pipe opening over them, and we find continual frost and winter. and, at the same time, by the draught thus created to remove It remains for us now to see how the princi. the foul air from the upper part of the room.

ples we have examined explain the complicated The usual defect in ventilation is that this foul air is allowed phenomena of the winds. to accumulate near the ceiling. If we stand on steps so as to Wind is merely air in motion ; but this mohave our head near the top of a room with several people and tion varies from time to time very greatly, both lighted gas-burners in it, we shall be fully convinced of this in direction and in intensity. There are various

modes of measuring the speed of the wind. A As a knowledge of the fact that heated air always ascends is rough idea of it may sometimes be formed by of great importance to us in understanding the theory of the noticing the shadows of clouds passing along wind, we will mention another simple illustration of it. Take a the ground; but this is vague and uncertain. circular piece of card, about six or eight inches in diameter, and, Various machines, known as anemometers, have beginning at the exterior, cut it spirally inwards so as to form a therefore been contrived for this purpose, and strip about 1 of an inch wide. Stick a pin through

the centre, are in use in most observatories. As the pressure and by this suspend it over a lighted gas-burner. The upward produoed on any surface varies as the square of Fig. 21. current of heated air will cause it to turn rapidly round, and it the velocity, some of the simplest anemometers will somewhat resemble a serpent. The old smoke-jack used to merely show the pressure, and from this the velocity must be act in a similar way.

calculated by tables drawn up for the purpose. Fig. 21 repreIf we open the door of a warm room a very little way, and sents one of the most common forms of this instrument, which hold a candle in the opening so as to observe the direction of is known as Lind's anemometer. It consists of a glass tube bent the current by that of the flame, we shall see the mode in which into the shape of the letter U, but drawn out at the bend so as the air is changed. When the candle is held close to the floor, a to diminish the bore. The object of this is to moderate the strong inward current of air will be perceived. Being cold, it Auctuations which are cansed by the gusts of wind. One end is winks to the floor and there enters. As we raise the candle the also turned at right angles, and the

whole is suspended on current becomes weaker, till, about half-way up, it is quite pivot, so that this open end is always presented to the wind.

85

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fact.

VOL. IV.

small shield, placed over the end of the other limb, shelters it a | 1,000 miles per hour, while at 45°, north or south latitude

little from the wind, and it is found that as soon as the instrument is exposed to the wind, its pressure acts on the liquid and depresses it in the limb exposed to its force. The difference in the level of the water in the two limbs indicates the pressure. If this difference be one inch, the pressure is about 15lbs. per square foot. A metal disc, supported by a spiral spring, is likewise used at times to measure the pressure. Fans arranged like the sails of a mill, and giving motion to a registering apparatus, have also been tried; but these forms have, for the most part, given way to that known as Robinson's anemometer. He found that the vertical vanes moved much more slowly than the wind, and therefore substituted for them metal cups or hemispheres. Four of these, with their open sides turned the same way, are accordingly mounted at the extremities of two arms crossing at right angles, and the axle to which these are fixed is connected with a train of wheels so as to register the revolutions. When this instrument is placed in an exposed place, the wind acts on the concave sides presented to it, and thus sets the whole in rotation, the speed of the cups being nearly equal to that of the wind, and always bearing the same proportion to it. This instrument is often fitted up by the side ef a vane, and both are made self-registering. The construction of the apparatus for this is very ingenious, but its description would be too long for insertion here. The following table gives an idea of the effects produced by different velocities of the wind –

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Winds vary in direction as much as invelocity. In temperate climates they are often proverbially uncertain, but in the tropics they are very much more regular; and we will now notice a few of them, which are distinguished by special names on account of their importance.

The most important are the trade-winds. These are permament, and extend a few degrees north and south of the tropics, the actual limits varying with the season. Their direction in the northern hemisphere is from the north-east, and in the southern from the south-east. Hence they are called the northeast and south-east trades. Between them there exists a narrow region of calms sometimes called the Doldrums. This band is a little north of the equator, and nearly, corresponds with the line of greatest heat on the earth.

Columbus was the first who noticed these winds; and when the sailors found that a constant wind kept on day after day, taking them further from their home, they almost mutinied. So regular are they that a ship will often sail from the neighbourhood of the Canaries right across to the north coast of South America without altering her sails. A little consideration will render the origin of these winds perfectly clear. In tropical regions the surface of the earth, being exposed to the almost vertical rays of the sun, becomes intensely heated, and communicates this high temperature to the air around. This accordingly expands and rises, its place being supplied by colder air, which rushes from temperate and polar regions. Now we should at first expect that this would cause the wind to blow from both poles towards the equator—that is, a north wind in the northern hemisphere and a south wind in the southern (for a wind is named after the point from which it blows); and if the earth were at rest, this would be the case. It is not at rest, however, but rotating rapidly from west to east; and it is clear that since the diameter of the earth is greatest at the equator, and diminishes as we recede from it, the velocity too must be greatest there. +- ‘lace at the equator has a velocity of nearly

it is only 660. The air, therefore, as it flows towards the equator, has at every point a less velocity than that of the portion of the earth's surface it is moving over, and hence it is left behind, and appears to be moving in a contrary direction to the earth. Now the motion of the earth is from west to east; the wind therefore appears to travel from east to west, and this, combined with its motion towards the equator, produces the north-east and south-east trade-winds. These and other regular winds are best observed in the ocean far away fromland, as that always interferes, more or less, by creating fresh currents. It is said that a difference is caused in the wind by the proximity even of a coral reef or small island. Now it is manifest that there must be some return currentfor the air, as otherwise it could not continue to flow constantly in the same direction, and it is found that this return current is above the ordinary one, and is in the contrary direction. It is difficult to afford full proof of this, but several facts show that such is the case. Rather more than fifty years ago an eruption took place of the volcano in St. Vincent, and a large quantity of ashes and scoriae was thrown out to a considerable elevation. A portion of this fell and covered the island of Barbadoes. Now the trade-wind blows directly away from Barbadoes; and it seems certain, therefore, that the ashes were thrown up high enough to reach the upper or return current, and were carried along with that. Humboldt states, likewise, that when he ascended the Peak of Teneriffe he found a very strong west wind, while the ordinary north-east trade-wind was blowing on the earth below at the same time. We see, then, that there is a system of convection being carried on in the air on a gigantic scale, the air, heated by the tropical sun, ascending and making room for colder air from temperate regions. As the fact of the sun being vertical over parts of the torrid zone is thus the cause of these winds, we should expect to find that when the sun is north of the equator they extend further north than when the sun is in the southern hemisphere, and such is really the case. The variations in their limits thus caused are usually laid down on maps which show the prevailing winds. After the current of heated air that ascends from the equator has travelled some distance, it becomes cooled down by radiation into space, so that its temperature falls below that of the under current. They therefore change places, and the return current from the equator blows on the surface of the earth, producing south-west winds in the northern hemisphere, and north-west in the southern. The west direction of these arises from the is: that they come from a portion of the earth where the motion is more rapid, and they therefore travel faster than the part they blow over, The latitude at which this current descends is about 30°. It varies, however, considerably in different parts owing to local causes, and therefore the winds in the temperate zone have not the same degree of regularity as they have nearer the equator We find, however, by records kept at various places, that if * measure the duration and intensity of the wind in places in the north temperate zone, the prevailing direction is south-west to south-south-west. Still further north, this current, which has become heated by contact with the earth, again ascends wo becomes the upper current. | In the Indian Ocean the effect of the trade-winds is bo little felt, but they are replaced by the monsoons. These so periodical winds, and extend from a few degrees south of to equator to the northern shores of the Indian Ocean. Their direction from April to October is south-west, and from Octobe to April north-east. At the periods of change violent storm are experienced, by which great damage is frequently done to shipping and property. This period usually lasts nearly month, The explanation of these phenomena is found in the fact to when the sun comes north of the equator, the plains of India an the surrounding countries become intensely heated by the almos vertical rays of the sun. The air over them, therefore, become rarefied and ascends, creating a strong current towards tho parts from the southward, for the sea does not become so heats by the sun's rays as the land does, but preserves an almost un form temperature. When the sun crosses the equator sonto ward, the plains of Australia and the islands near it becom

similarly heated, and thus a reverse current is produced. The winds are, however, somewhat modified by the nature of the country and other causes, which are not yet fully understood. For purposes of commerce they are found almost more useful than the trade-winds, as a vessel can proceed to a port during one monsoon, wait there to discharge her cargo, and re-load in the interval of the change, and return with the contrary wind. The land and sea breezes are the next winds which we must notice. These are more felt in lower latitudes than in ours; but most visitors to the seaside in our own country have doubtless noticed them. A short time after sunset a gentle breeze sets off the coast towards the sea, and continues to blow during the evening and night. After the sun has risen again, this wind ceases, and a contrary one sets in from the sea, mitigating the intense heat which would otherwise be felt. This breeze continues during the day. These phenomena are said to be well seen in the neighbourhood of Vesuvius, where during the day the column of smoke blows inland, but at evening it veers round till it points exactly in the contrary direction. These winds are easily accounted for. The sea, as we have already remarked, preserves a nearly uniform temperature, being little affected by the heat of the sun. The land, on the contrary, becomes rapidly heated by day, and at night parts with its heat as rapidly by radiation. The consequence is that, as soon as the sun has been shining a little time, the land attains a higher temperature than the sea; the air over it therefore rises, and colder air flows in from the sea to fill its place. In the evening this extra heat soon passes off, and the land becomes colder than the sea. The contrary effect then ensues: the air rises over the sea, and a breeze springs up from the land. A simple experiment will serve well to illustrate these phenomena. Fill a tub or other large vessel with cold water, and float on it a saucer filled with warm water. This will represent the island when heated during the day. If we now hold a smoking taper or piece of paper near it, we shall see that a current of air is setting in towards the saucer. Now reverse the experiment, and fill the large vessel with warm water and the small with cold, and we shall find that the current of air flows over the edge of the saucer and drives the smoke away, just as at evening the wind sets off the land. There are many other periodical winds, but as for the most part they are confined to small areas and only known by local names, we need not further allude to them, except to say that nearly all may be accounted for on the same principles as those we have been considering. The winds frequently met with on the borders of desert regions, and known in Arabia as the Simoom, in the neighbourhood of the Mediterranean as the Sirocco, and by other names in other places, owe most of their dangerous character to their dryness, and to the amount of fine dust they carry with them. Sweeping over large tracts of arid sand, they become intensely hot and dry, and often cause the death of those who are unable to obtain shelter from their influence. If we thus trace the quarter from which any wind blows, we shall usually be able to understand the reason of its special eharacter. Thus, in England, rain usually comes with a southwest wind, the reason being that this wind, coming from lower Latitudes, is warm, and therefore absorbs much water as it passes over the sea. When it reaches our shores it becomes cooled, and a portion of its vapour is at once condensed and falls in the form of rain. In mountainous districts a current of air frequently impinges against the cold tops of a mountain, and becoming in this way ddenly cooled, rushes violently down the side, often giving rise to violent and destructive hurricanes. The Pamperos, or wirids which rush from the Andes with great violence across the plains of South America, owe their origin to this cause; and in our own lake districts in Cumberland a somewhat airoilar effect, though on a much smaller scale, may at times be rocyticed. The whole subject of the varying phenomena of the atmosphere is one of great importance and interest; but we cannot enlarge upon it here. Enough, however, has, we hope, been said to explain fully the romain principles, and to stimulate the student to make further researches, and we must therefore bring our lessons on Pneuratics to a close. In our next paper on Natural Philosophy we

ANSWER TO EXAMPLES IN LESSON V. (Page 35.)

1. The pressure is 5 x 6.25 x 29:04 x 0.491 pounds, which is equal to 445-5825 pounds.

2. Since the volume occupied by the air is only #, i.e. * of its eriginal volume, the pressure is increased *. The mercury therefore stands #9 x 29 inches, or 64; above the level in the other limb, or 09: above 0. 3, 238-21 cubic inches. 4. Just over 1461 cubic inches. 5. The temperature to which it is raised is 338°. 6. 128-09 cubic inches.

CORRESPONDENCE IN FRENCH.-II.

6.-REPLY TO LETTER OF INQUIRY As To Solvency of A FIRM. Paris, August 20, 1866. Messrs. Merivale Brothers, Havre, Gentlemen, We beg to acknowledge the receipt of your letters of the 26th of July and 19th inst. Although we say that the firm H. Smith Bros. is sound, it is but fair to tell you, in confidence, that their reputation is not entirely above all suspicion; that this suspicion attaches itself chiefly to a want of delicacy and scruple in their business, of which a number of people have from time to time raised complaints, so as to diminish the credit which the firm used to enjoy. We cannot, however, limit the extent of credit you may be disposed to give them, except in so far the foregoing may determine you. Please to send us particulars as to what is being done in quinine in your city, and whether, as with us, it seems to go down. We are offered some of superior quality at 4 francs 50 cents. Be good enough to send us all the particulars you can obtain of this drug, partly as to the monopoly, as to the news from Peru, especially about the war, and as to the most favourable time for purchase. We are, Gentlemen, your very obedient servants, HENRY L.AFITTE & Co.

Paris, le 20 Aosit, 1866. Messieurs Merivale Frères, au Havre. Messieurs, Nous accusons réception de vos lettres du 26 Juillet et du 19 courant. Tout en vous disant que la maison H. Smith freres est; solide, nous devons aussi vous informer confidentiellement qu’elle me jouit pas entièrement d'une réputation libre de toute censure; que cette censure porte sur un manque de délicatesse et de scrupule dans leurs affaires, dont bon nombre d'individus se plaignent, de manière à diminuer le crédit dont jouissait autrefois cette maison. Nous ne pourrions donc vous fixer sur l'étendue du crédit A leur accorder sinon par ce que nous vemons de dire. Veuillez bien nous informer comment va le commerce de la quinine sur votre place et nous dire si, comme ici, cet article semble dévoir fléchir. On nous en offre, de belle et bonne qualité, a 4fr. 50c. Ayez la bonté de recueillir sur cet article tous les renseignements que vous serez à même devous procurer, tant sur le monopole, que surles nouvelles qui arrivent du Pérou, au sujet de la guerre, et sur le moment le plus favorable pour faire des achats. Agréez, Messieurs, l'assurance denotre parfaite considération, HENRI LAFITTE & Co. 7.—LETTER of INQUIRY As To STATE of MARRETS UNDER ADVERSE CIRCUMSTANCEs. Amsterdam, May 2nd, 1861. Messrs. J. T. Wan Praat & Co., London. Gentlemen, The latest news from America has overcast the commercial horizon and created a panic in our city. We tremble for the consequences which such a state of things may produce, The best houses are shaking, and we are momentarily expecting a general suspension. We do not know how your markets are, and in any case it would not become as to give you any advice; we merely throw out a hint, that the consequences of these untoward circumstances must be felt, with you, as in all the great industrial centres. Be good enough to give us some information as to what you

saa A commence the study of Electricity.

are doing, what you fear or hope, for we are somewhat uneasy

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hour.

2 5 10

0.123

35

6.027

12.300

small shield, placed over the end of the other limb, shelters it a 1,000 miles per hour, while at 45°, north or south latitude little from the wind, and it is found that as soon as the instru- it is only 660. The air, therefore, as it flows towards the ment is exposed to the wind, its pressure acts on the liquid and equator, has at every point a less velocity than that of the depresses it in the limb exposed to its force. The difference in portion of the earth's surface it is moving over, and hence it is the level of the water in the two limbs indicates the pressure. left behind, and appears to be moving in a contrary direction to If this difference be one inch, the pressure is about 15lbs. per the earth. Now the motion of the earth is from west to east; square foot.

the wind therefore appears to travel from east to west, and this, A metal disc, supported by a spiral spring, is likewise nsed combined with its motion towards the equator, produces the at times to measure the pressure. Fans arranged like the sails north-east and south-east trade-winds. These and other regular of a mill, and giving motion to a registering apparatus, have also winds are best observed in the ocean far away from land, as that been tried; but these forms have, for the most part, given way always interferes, more or less, by creating fresh currents. It to that known as Robinson's anemometer.

is said that a difference is caused in the wind by the proximity He found that the vertical vanes moved much more slowly even of a coral reef or small island. than the wind, and therefore substituted for them metal cups or Now it is manifest that there must be some return current for hemispheres. Four of these, with their open sides turned the the air, as otherwise it could not continue to flow constantly in same way, are accordingly mounted at the extremities of two the same direction, and it is found that this return current is arms crossing at right angles, and the axle to which these are above the ordinary one, and is in the contrary direction. It is fixed is connected with a train of wheels so as to register the difficult to afford full proof of this, but several facts show that revolutions.

such is the case. Rather more than fifty years ago an eruption When this instrument is placed in an exposed place, the wind took place of the volcano in St. Vincent, and a large quantity of acts on the concave sides presented to it, and thus sets the ashes and scoriæ was thrown out to a considerable elevation. A whole in rotation, the speed of the cups being nearly equal to portion of this fell and covered the island of Barbadoes. Now that of the wind, and always bearing the same proportion to it. the trade-wind blows directly away from Barbadoes; and it This instrument is often fitted up by the side of a vane, and seems certain, therefore, that the ashes were thrown up high both are made self-registering. The construction of the enough to reach the upper or return current, and were carried apparatus for this is very ingenious, but its description would along with that. be too long for insertion here.

Humboldt states, likewise, that when he ascended the Peak of The following table gives an idea of the effects produced by Teneriffe he found a very strong west wind, while the ordinary different velocities of the wind :

north-east trade-wind was blowing on the earth below at the same Mlles per Pressure per square

time. We see, then, that there is a system of convection being foot in pounds. Characteristics.

carried on in the air on a gigantic scale, the air, heated by the 1 0.005

Hardly perceptible. tropical sun, ascending and making room for colder air from 0.020 Just perceptible.

temperate regions. As the fact of the sun being vertical over Gentle breeze. 0-492

parts of the torrid zone is thus the cause of these winds, we

Brisk wind. 20 1.968

should expect to find that when the sun is north of the equator 25

3.075
Very brisk wind.

they extend further north than when the sun is in the southern High wind,

hemisphere, and such is really the case. The variations in their 45

96963
Very high wind.

limits thus caused are usually laid down on maps which shos 50

Storm.

the prevailing winds. 60 17.715 Great storm,

After the current of heated air that ascends from the equator 80 31.490 Hurricane.

.

[graphic]

has travelled some distance, it becomes cooled down by radiation 100 49.200

Great hurricane, carrying into space, so that its temperature falls below that of the undertrees, etc., before it.

current. They therefore change places, and the return current Winds vary in direction as much as in velocity. In temperate from the equator blows on the surface of the earth, producing climates they are often proverbially uncertain, but in the tropics south-west winds in the northern hemisphere, and north-west in they are very much more regular; and we will now notice a few the southern. The west direction of these arises from the fact of them, which are distinguished by special names on account of that they come from a portion of the earth where the motion is their importance.

more rapid, and they therefore travel faster than the part they The most important are the trade-winds. These are perma- blow over. nent, and extend a few degrees north and south of the tropics, The latitude at which this current descends is about 30°. It the actual limits varying with the season. Their direction in variés, however, considerably in different parts owing to local the northern hemisphere is from the north-east, and in the causes, and therefore the winds in the temperate zone have not southern from the south-east. Hence they are called the north the same degree of regularity as they have nearer the equator. east and south-east trades. Between them there exists a We find, however, by records kept at various places, that if we narrow region of calms sometimes called the Doldrums. This measure the duration and intensity of the wind in places in the band is a little north of the equator, and nearly. corresponds north temperate zone, the prevailing direction is south-west or with the line of greatest heat on the earth.

south-south-west. Still further north, this current, which has Columbus was the first who noticed these winds ; and when become heated by contact with the earth, again ascends and the sailors found that a constant wind kept on day after day, becomes the upper current. taking them further from their home, they almost mutinied. In the Indian Ocean the effect of the trade-winds is bat So regular are they that a ship will often sail from the neigh- little felt, but they are replaced by the monsoons. These ant bourhood of the Canaries right across to the north coast of South periodical winds, and extend from a few degrees south of the America without altering her sails. A little consideration will equator to the northern shores of the Indian Ocean. render the origin of these winds perfectly clear. In tropical direction from April to October is south-west, and from October regions the surface of the earth, being exposed to the almost to April north-east. At the periods of change violent storms vertical rays of the sun, becomes intensely heated, and communi- are experienced, by which great damage is frequently done to cates this high temperature to the air around. This accordingly shipping and property. This period usually lasts nearly 3 expands and rises, its place being supplied by colder air, which month. rushes from temperate and polar regions. Now we should at The explanation of these phenomena is found in the fact that first expect that this would cause the wind to blow from both when the sun comes north of the eqnator, the plains of India and poles towards the equator-that is, a north wind

in the northern the surrounding countries become intensely heated by the almost hemisphere and a south wind in the

southern (for a wind is vertical rays of the sun. The air over them, therefore, becomes named after the point from which

it blows); and if the earth were rarefied and ascends, creating a strong current towards those at rest,

this would be the case. It is not at rest, however, but parts from the southward, for the sea does not become so heate rotating rapidly from west to east; and

it is clear that since the by the sun's rays as the land does, but preserves an almost unidiameter of the earth is greatest at the eqnator, and diminishes form temperature. When the san crosses the equator southas we recede from it, the velocity too must be greatest there ward, the plains of Australia and the islands near it become We find that any place at the equator has a velocity of nearly similarly heated, and thus a reverse current is produced. These

Their

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