substance as wood or iron; for no force can bring the sides together, without breaking the bladder, although the parts of an empty bladder may be squeezed into any shape. The same thing is proved by the following experiment:-Open a pair of common bellows, and then stop the nozle, so that no air can rush out-and no force whatever can bring the parts together, without bursting the leather, or unstopping the nozle. That heat increases the elasticity of air, may be shown, by placing before a strong fire a bladder with a small quantity of air, when the small portion of air will expand, till the bladder appear quite full and ready to burst. These experiments may be applied to the explanation of such phenomena as the following:-Why the compressed air between the liquid and the cork, in a bottle of beer or ale, bursts forth in the form of froth when the cork is drawn-why fishes, in consequence of their air-bladders, are enabled to rise and sink in the water-and why the carcass of a man that has been drowned, in a few days rises and floats on the surface for a short time, and then sinks to rise no more. The compressibility of air may be shown, by taking a glass tube which is open only at one end, and of course full of air, and plunging the open end into a vessel of water, when the water will be seen to have risen to a small height, near the bottom of the tube, which proves that the air which filled the whole length of the tube is compressed by the water, into a smaller space. In a similar Iway the principle of the diving-bell may be illustrated. Let A B, Fig. 1, represent a large tumbler or drinking glass, which may be nearly filled with water. Place a piece of cork on the surface of the water, and over the cork an ale-glass C D, with its mouth downwards, then push the glass perpendicularly down towards the bottom of the tumbler, and

with safety in the depths of the sea without touching the water, provided fresh air be supplied. A small quantity of water will be found to have entered the ale-glass, and the deeper it is plunged in any vessel the higher will the water rise within it. At the depth of 33 feet, where the pressure of the atmosphere is doubled, a diving-bell will be half filled with water-at the depth of 66 feet, it will be two-thirds filled-at the depth of 99 feet, it will be three-fourths filled, and so on in proportion to the depth; which shows the propriety of having this vessel in the form of a bell, that the perpendicular height of the water may be as little as possible. The following simple experiment illustrates the pressure of the atmosphere in a mode somewhat different from those already stated. Procure a tin vessel about six or seven inches long, and three in diameter, having its mouth about a quarter of an inch wide, as E F, Fig. 2. In its bottom make a number of small holes, about the diameter of a common sewingneedle. Plunge this vessel in water, and when full cork it up, so that no air can enter at the top. So long as it remains corked, no water will run out-the pressure of the atmosphere at the bottom preventing it; but as soon as it is uncorked, the water will issue from the small holes in the bottom, by the pressure of the air from above. The same experiment may be made by means of a tube, Fig. 2.

E

F

Fig. 3

H

water, in a quarry or other situation, which is wished to be drained, and where there is no declivity or lower ground adjacent to which the water can be conveyed-it may be carried over the rising ground M N, by means of the syphon M N L; provided the perpendicular elevation N P above the level of the pool M, does not exceed thirty-two feet, for to that height only will the water rise in the syphon by the pressure of the atmosphere; and provided that the end of the syphon at L descends a little way beyond the level of the pool at M,-in which case, when the syphon is filled, the water will rush out at L, so long as any remains in the pond. In the same way may be shown how a cask of liquor may be decanted by a syphon placed in a hole made in its upper side. The use of the syphon might likewise be shown when placed in a reverse position, as in Fig. 5, when it

R

Fig. 5.

may be applied to the purpose of conveying water from a fountain at R, along a hollow or valley to a house, S, at the same height on the other side of the valley; and however deep or broad the valley may be, the water may in this manner be conveyed, provided the syphon is sufficiently strong near its lower parts to sustain the perpendicular pressure of the water.

The following simple and interesting experiment might be exhibited to show the effects of the expansion of air. Procure a common Florence flask, F G, Fig. 6, and pour into it a large wine-glassfull of water; then take a tube, I H, bent at the top, H, like a small syphon, and fasten it air-tight into the mouth

Fig. 6.

H

of the flask, I, so that its bottom may be immersed in the water at K, but not touching the bottom of the flask. Then immerse the flask into a vessel of very hot water, when in consequence of the expansion of the air in the flask, the water at K will be forced up into the tube I H, where it is received into a wineglass at H. Holding the wine-glass, into which the water is now received, at the end of the tube, as represented in the figure, take the flask out of the hot water, and plunge it into another vessel full of cold water, and the water in the wine- F glass will be thrown back into the bottom of the flask, by the pressure of the atmosphere on its surface at H. The flask may then be again immersed in the hot water, when the water at its bottom will be thrown up into the wine-glass, as before, and the operations may be repeated as often as judged expedient. This experiment when dexterously performed, seldom fails to produce a pleasing effect upon the spectators, especially when the water is tinged with a red colour, by means of the sulphuric or any other acid dropped into an infusion of red cabbage.*

The science of Optics affords scope for

* In arranging and performing such simple experiments as those above stated, it is expedient that the teacher or operator should know how to cut phials and glass tubes, and to form syphons. The neck of a common phial may be cut off so as to form a tube, by slightly indenting a portion of the circumference with the sharp edge of a common file, and then, with the point of a hot iron, beginning at the indention, go round the circumference of the phial, and the head will at once be separated from the body. Otherwise, tie a thread which has been steeped in turpentine or spirits of wine, firmly round the mouth of the phial, then set fire to it, and the operation is performed In the same manner, long glass tubes may be cut into any lengths. If the tubes be of a small diameter, it is only requisite to indent them with a file at then holding one end of the tube in the left hand, the point where they are intended to be cut, and give a blow with the right on the other end, and the tube will snap asunder. To bend a glass tube into the form of a syphon: Put the tube through the bars of a common grate, when the fire is burning clear; let the part of the tube which is to he bent be in the centre or hottest part of the fire; take hold of the tube at both ends, and when it begins to melt near the middle, gently bend it with both hands, in the form which is wanted, and then render such operations quite easy and efficient for remove it from the fire. A little experience will the purpose intended. If a small bend only at one end of the tube is required, that end may be put into the fire till it begin to melt, then take hold of it gently with a pair of tongs, and bend it in the form required with the right hand.

from E F. The rays proceeding from the lens A B, and falling upon the speculum, are reflected in a perpendicular direction to the lens EF, where they enter the eye, which looks down upon the object through the side of the tube. When this eye-piece is applied to a telescope, with the lens E F on the upper part of it, we look down upon the object as if it were under our feet. If we turn the eyepiece round in its socket a quarter of a circle towards the left, an object directly before us in the south will appear as if it were in the west, and turned upside down. If from this position, it is turned round a semicircle to wards the right, and the eye applied, the same object will appear as if it were situated in the east; and if it be turned round another quadrant, till it be directly opposite to its first position, and the eye applied from below, the object or landscape will appear as if suspended in the atmosphere above us. Such experiments, when accompanied with proper diagrams, and an explanation of optical principles, may easily be rendered both entertaining and instructive. A camera obFig. 3. scura, on a larger scale, and on a different plan from that alluded to above, might be erected on the top of every schoolhouse, which is constructed with a flat roof, as formerly suggested. Fig. 3 contains a re

K

T

H

presentation of a wooden building, on the top of which is a large convex lens H I, about 10 or 12 feet focal distance. At half a right angle to this lens is a plain speculum, by which the rays of light from the objects O are reflected downwards through the lens, which forms a picture of all the objects before the speculum, on a round white table, T, in all their colours, motions, and proportions. If

the speculum be made to revolve, the whole of the surrounding landscape may be successively depicted on the table. When the lens is of a long focal distance, as from 10 to 15 or 20 feet, it produces a pretty powerful telescopical effect, so that objects may be distinctly perceived at a considerable distance, and individuals recognized on the picture at the distance of a mile or more. Wherever there are objects in motion, such as ships sailing, birds flying, smoke ascending, crowds of people moving to and fro, or boys and girls engaged in their amusements; this exhibition always affords a high degree of satisfaction. It might occasionally be used, not only as an illustration of optical principles, but also as a reward for diligence and good behaviour.

In connection with the above, representations might be given of natural and artificial objects as exhibited by the phantasmagoria. Discarding the ridiculous and childish figures which were formerly used in the common magic lanterns, opticians have now constructed sliders which exhibit representations of the telescopic appearances of the heavenly bodies, the different constellations, the motions of the earth and moon, and various objects connected with botany, mineralogy, and zoology; and such objects, when exhibited in this manner, are calculated to produce both instruction and amusement. The solar microscope in particular, (or the oxy-hydrogen, if it can be procured,) should be occasionally exhibited to the young, to convey to them some ideas of the wonderful minuteness of the atoms of matter, and the admirable mechanism displayed in the structure of vegetables and the bodies of animals, particularly in those myriads

of animalcula which are invisible to the unassisted eye. Such animalculæ may be procured almost at any season, but particularly during the summer months, by infusing in separate open vessels, small bits of grass or hay, leaves of flowers, or other vegetable substances, when, after a week or ten days, animalculæ of different kinds, according to the nature of the substances infused, will be perceived in vast numbers, by the aid of the microscope, in every drop of the infusion. A compound microscope is perhaps as good an instrument as any other for giving a steady and satisfactory view of such objects; and the only objection to its use for a school is, that only one individual can see the object at a time. When a teacher is not furnished with an instrument of this kind fitted up in the usual way, he may, with a little trouble, construct a compound microscope, by means of the eye-piece of a common pocket achromatic telescope, which may be purchased for one guinea, or less. The eye-pieces of such telescopes contain four glasses arranged on a

principle somewhat similar to that of the glasses of a compound microscope. If we screw off one of these eye-pieces, and look through it in the usual way, holding the object end about a quarter of an inch distant from any small object, such as the letters of a printed book, it will appear magnified about ten or twelve times in length and breadth; remove from the tube the third glass from the eye, which is the second from the object, and look through it in the same manner, holding it more than an inch distant from the object, and it will appear magnified more than twenty times in diameter, or above 400 times in surface. If, by means of small pasteboard tubes, or any other contrivance, we attach the glass that was taken out of the outside of the objectglass of the eye-piece, so as to be nearly close to it, we shall have a magnifying power of nearly forty times; or, if we substitute for these two object-glasses a single glass of about a half-inch focal distance, we shall form a pretty good compound microscope, magnifying above forty times in diameter, and 1600 times in surface, which will afford very pleas ing views of various objects in the animal and vegetable kingdoms. The magnifying powers now stated will differ somewhat in different eye-pieces, according to their lengths and the focal distances of the glasses of which they are composed. The tube of the eye-piece thus arranged, may be occasionally fitted into a pasteboard tube supported by three pillars, in which it may be moved up or down for adjusting it to distinct vision, and the object placed underneath and properly illuminated. These hints are suggested on the score of economy, for those who have no regular microscopic apparatus.

Various amusing experiments besides the above might be exhibited to the young, such as the optical paradox, an instrument through which objects may be seen, although a board or other opaque body be interposed between the eye and the objects—the prism, which, in a dark room, separates the primary colours of the solar rays the multiplying glass, which makes one object appear as if there were ten, twenty, or thirty-the burning-glass, which, by means of the sun's rays, sets on fire dark coloured paper, wood, and other inflammable substances and optical illusions produced by the various refractions and reflections of light in water, combinations of plane mirrors, and by concave speculums. A concave mirror, about 5 or 6 inches diameter, and 10 or 12 inches focus, which may be procured for about half-a-guinea or 15 shillings, is of great utility for a variety of exhibitions. 1. When held at nearly its focal distance from one's face, it represents it as magnified to a monstrous size. 2. When held in the solar rays,

directly opposite the sun, it collects the rays into a focus before it, so as to act as a powerful burning-glass, and in this way a hole may be burned in a thin board. 3. When hung at an elevation of about 5 feet, and a person placed opposite to it, at 6 or 7 feet distant, he will see his image hanging in the air in an inverted position, between him and the mirror, and if he approach a little nearer the mirror, and hold out his hand towards it, the image will appear to do the same, as if about to shake hands, and if he stretch his hand still nearer the mirror, the hand of his image will appear to pass by his hand, and approach nearer his body. 4. Such a mirror is of use in explaining the construction of a reflecting telescope. When it is held opposite to a window, the image of the sash and of the objects without the window will be seen depicted in its focus on a piece of white paper held between it and the window, which represents the manner in which the first image is formed by the great mirror of a reflecting telescope;and the manner in which the small speculum of a Gregorian reflector forms the second image, may be shown by holding the mirror at a little more than its focal distance behind a candle, and throwing its magnified image upon an opposite wall, in the same way as the lens, fig. 1, p. 122, by refraction, produced the enlarged image C D. 5. If a bright fire be made in a large room, and a very smooth, well-polished mahogany table be placed at a considerable distance near the wall, and the concave mirror so placed that the light of the fire may be reflected from the mirror to its focus on the table-a person standing at a distance toward the fire, but not directly in the line between the mirror and the fire, will see an image of the fire upon the table, large and erect, as if the table had been set on fire.

Various illusions and deceptions have been produced by means of concave mirrors. Pagan priests are supposed to have rekindled the Vestal fire by this instrument; and with the same instrument, on a large scale, Archimedes is reported to have burned the Roman fleet. When the mirror is concealed from the view of a spectator by certain contrivances, he may be easily deceived and tantalized with a shadow instead of a substance. He may be made to see a vessel half full of water inverted in the air without losing a drop of its contents. He may be desired to grasp what appears a beautiful flower, and, when he attempts to touch it, it vanishes into air, or a death's-head appears to snap at his fingers. He may be made to behold a terrific spectre suddenly starting up before him, or a person with a drawn sword, as if about to run him through. An exhibition of this kind was some time ago brought before the public, which was effected by a