1

various useful and entertaining experiments may be exhibited by means of a simple apparatus which almost every one can procure. For example, the pressure of the atmosphere may be proved to the conviction of every one by such simple experiments as the following:-The common experiment of filling a wine-glass with water, covering its mouth with a piece of paper, and then inverting it, is quite decisive of the atmospheric pressure; for the paper underneath, instead of being convex by the pressure of the water within, is concave, by the pressure of the atmosphere from without; and no other cause can be assigned why the water is supported in the glass. Another simple experiment, where no paper is employed, proves the same fact: Take a glass tube, two or three feet long, with a narrow bore; put one end of it into a vessel of water, put your mouth to the other end and make a deep inspiration till the air is drawn out of the tube, when the water will rush to the top of the tube; then place your thumb on the top to prevent the access of air from above, and when the other end of the tube is taken out of the water, the column of water will be suspended in the tube by the atmospheric pressure, although the lower end of it is open. When the air is sucked out of the tube, a vacuum is produced, and the external air, pressing upon the surface of the water in the vessel, forces it to the top of the tube; the thumb being applied prevents the air pressing the water down, and the atmospheric pressure on the bottom prevents the water from running out. The same fact is proved by the following experiment: Let a piece of burning paper be put into a wine-glass, so as to rarify or exhaust the air, and while it is still burning, press the palm of the hand against the mouth of the glass, when it will adhere with a considerable degree of force, by the pressure of the atmosphere on the bottom and sides of the glass. This experiment may be varied as follows: Pour a certain quantity of water into a saucer; invert a wineglass over a piece of burning paper, or burning Brandy, and, after hoiding it a short time in the flame, place it in the saucer, when the water will rush up into the glass in consequence of the atmospheric pressure, as it did in the glass tube when it was exhausted of its air by suction. These and similar experiments, which every one may perform, are as decisive proofs of the atmospheric pressure as those which are performed by means of the airpump. Such experiments, when conducted by intelligent teachers, may easily be applied to the explanation of the causes of certain natural and artificial processes, such as the firm adherence of two polished surfaces-the action of a boy's sucker in lifting large stones-the operation of cupping-the process of a child sucking its mother's breast-the effects produced by cements-:he rise of water in pumps-the firm adhesion of snails and shell-fish to rocks and stones-the action of syphons-what is termed suction, as when we take a draught of water from a running stream-the fact, that a cask will not run, in certain cases, unless an opening is made in its top-and many similar processes, some of which will be found of considerable practical utility. The elasticity of the air may be proved by such experiments as these:-Take a bladder, and fill it with air by blowing into it, and then apply a force to the sides of it, so as to compress it into a smaller space; when the force is removed it immediately expands and fills the same space as before. This experiment proves, not only the elasticity of air, but that, though invisible, it is as much a material 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,

85

2.

mouth about a quarter of an inch wide, as E F, Fig. but as soon as it is uncorked, the water will issue In its bottom make a number of small holes, from the small holes in the bottom, by the pressure about the diameter of a common sewing-needle.of the air from above. The same experiment may

be made by means of a tube, seven or eight inches long, and about three-fourths of an inch diameter, having two or three small holes in its bottom; and another tube, GH, Fig. 3, of the same dimensions, having a small hole in each side, I K, will illustrate the lateral pressure of the atmosphere-the water being retained when it is corked, and running out when the cork is removed. It will likewise illus trate the lateral pressure of water and other liquids.

Several amusing experiments may also be performed by means of syphons, when concealed in drinking-cups and other vessels; and the utility of the principle on which they act may be illustrated in certain practical operations. For example, their use may be shown in conveying water over a rising ground. In Fig. 4, let M represent a pond or pool of 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 Fig. 4.

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;

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.

sion of air. Procure a common Florence flask, FG, Fig. 6, and pour into it a large wine-glassfull of wa ter; then take a tube, I H, bent at the top, H, like a small syphon, and fasten it air-tight into the mouth 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 The following simple and interesting experiment very hot water, when, in consequence of the expanmight be exhibited to show the effects 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 wine-glass 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-glass will be thrown back into the bottom of the flask, by the pressure of the atmosphere on its surface at H. Fig. 6.

1

H

will be formed a large magnified image of the can. dle, CE D. This image will be inverted, and larger than the flame of the candle in proportion as the distance A E, from the glass to the wall, exceeds the distance A B, from the glass to the candle. Suppose the distance A B to be exactly 6 inches, and the distance A E to be 7 feet or 84 inches, then the image of the candle will be magnified in the proportion of 6 to 84, or 14 times. In this experiment the candle represents the object to be magnified in a compound microscope, A the object-glass, and C D the image formed by the lens, which is magnified a second time by the eye-glass of the microscope. In reference to the solar microscope, the candle represents the small object to be magnified, and CD its magnified image on a white wall or screen; and in reference to the magic lantern, or phantasmagoria, the candle represents the figures painted on the sliders, A the convex lens which throws the image of the figures on a screen, and CD the magnified image of the painted figures. In all these instru ments, the principle on which the objects are magnified is precisely the same; the size of the image is always in proportion to its distance from the lens by which it is formed; but as the image is enlarged it becomes less brilliant and distinct, and therefore there is a proper medium which must be fixed upon

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.

Fig. 3.

I

as to the distance between the lens and the screen on which the image is thrown; but a skilful teacher will always know how to modify such circumstances. The nature of a telescope and of the camera obscura may be illustrated as follows:-Fix a lens of 4, 5, or 6 feet focus, in a hole made in a windowshutter; darken the room, so that no light can enter but through the lens. If its focal distance be 5 feet, or 60 inches, a white screen placed at that distance A camera obscura, on a larger scale, and on a will receive the image of the objects without, oppo- different plan from that alluded to above, might be site the glass, where they will be beautifully depict- erected on the top of every school-house, which is ed in all their forms, colors, and motions, in an in- constructed with a flat roof, as formerly suggested. verted position, forming a kind of living picture.-Fig. 3 contains a representation of a wooden buildThis exhibition never fails to excite the admiration of the young. If now, a lens about 2 inches focus be placed 2 inches beyond the image thus formed, and the screen removed-in looking through this lens, the objects will appear magnified in the proportion of 2 inches to 60, that is, 30 times; and as ihe image was inverted, so the object, as seen through the glass, will appear as if turned upside down.This is perhaps one of the best modes of explaining the principle of a refracting telescope, and the reason why the object appears inverted, when viewed with a single eye-glass. The same thing may be partly shown by a common telescope. Having taken out all the eye-glasses, except the one next the eye, adjust the telescope to distinct vision, and all the objects seen through it will appear as if turned upside down. The manner in which the image is reversed by the other eye-glasses, and the object made to appear upright, might then be explained. Objects might likewise be exhibited through a telescope, as appearing in different positions and directions. This is effected by means of a diagonal eye-piece, which is constructed in the following manner: Let A B, Fig. 2, represent a convex glass about 2 inches focal

distance; C D a plain metallic speculum, of an oval form, well polished, and placed at half a right angle to the axis of the tube; and E F, another convex lens, 2 inches focus. The centre of the speculum may be about 1 inch from A B, and about inch from EF. The rays proceeding from the lens A B, and falling from the speculum, are reflected in a perpendicular direction to the lens E F, 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 EF on the upper part of it, we look down upon the object as if it were under our feet. If we turn the eye-piece 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 towards the right, and the eye applied, the same object will appear as if it were situated in

A lens is a round piece of glass, ground either concave or convex. All lenses that magnify objects are convex, or thicker in the middle than at the edge, such as common magnifiers, reading-glasses, and the glasses used in microscopes and telescopes, except the Galilean perspective, in which the eye-glass is concave.

H

T

ing, on the top of which is a large convex lens HI, 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 colors, motions, and proportions. If the speculum be made to revolve, the whole of the surrounding landscape may be succes sively 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 recognised 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 behavior.

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 ob jects, 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 exhi bited 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,