ratus, is highly desirable; as, without it, some of the most interesting experiments on this subject cannot be performed. But where this instrument, on account of its expense, cannot be procured, 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 wine-glass over a piece of burning paper or burning brandy, and, after holding 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 air-pump. 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 firin 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's sucking its mother's breast-the effects produced by cements-the 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, 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 carcase 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 way 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 CD, with its mouth downwards,

then push the glass perpendicularly down towards the bottom of the tumbler, and the cork will appear swimming a little above the bottom; plainly indicating that there is no water above it in the ale-glass, which is prevented from entering by the resistance of Fig. 1. C

B

the air within. The water in the tumbler may represent the water of a river or of the sea; the ale-glass may represent the divingbell, in which a person may sit 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 aleglass, 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 diamete of a common sewing-needle. 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 bot

tom, by the pressure 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 illustrate 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 illus. trated 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 Fig. 4. N

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 syphor. 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 may be applied to the purpose of conveying water Fig. 5.

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 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 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