CHAPTER VI.

THE STUDY.

'Hast thou no friend to set thy mind abroach?

Good sense will stagnate. Thoughts shut up want air,
And spoil, like bales unopened to the sun."-YOUNG.

In our last chapter we examined a few of the most prominent phenomena of the kitchen; now that we have arrived in the study, we shall be able to illustrate some of them still further; and as the pressure of the atmosphere is one of the great agents in producing many of the effects that will come under our consideration, we will devote a part of this chapter to the subject. And here I must apologise to my young reader for requiring somewhat more of his attention than I have yet demanded; for, though the subject is not very difficult to comprehend, it will require a considerable degree of thought to adapt it to all the various modes in which it will be presented to his view: he will, however, find himself well repaid for the time he may devote to it.

Atmospheric pressure is the weight of the air

resting on the surface of the earth; air extends to a height of many miles above the earth, and, although but a light fluid, yet so great an altitude of it must be of considerable weight. Were we to take a square tube, one inch in diameter inside, and so many miles high as to reach to the top of the atmosphere, the air in that tube would weigh about fifteen pounds: therefore every superficial inch of the earth's surface is pressed upon by a weight of fifteen pounds. Now air being a fluid, presses in all directions alike, consequently every surface which is exposed to its action, whether it be horizontal or perpendicular, or in whatever direction it may be placed, is subject to this pressure. The idea very naturally suggests itself, How is it that we are not sensible of this enormous weight, for fifteen pounds on every inch of the surface of a man's body would amount to many tons? The secret, however, consists in the pressure being exerted not only externally, but internally, and it is therefore balanced and rendered insensible to us; but, should any thing occur to disturb this equilibrium, we are immediately made sensible of the pressure. Suppose we take a sheet of writing paper stretched on a hollow frame, and press a book against it on one side, whilst another book is pressed against it on the other, no effect will be produced; but if we remove one book, the equilibrium is destroyed and the paper gives way under the pressure, which before

was imperceptible to it.

Just in the same manner

is the pressure of the atmosphere rendered imperceptible, whilst counterbalanced by a corresponding pressure on the opposite side.

If we place our hand over the open receiver of an air-pump and exhaust the air from beneath, we shall very quickly be made painfully sensible of this pressure; or if we tie a bladder over the glass, instead of placing the hand there, and again exhaust the air, the bladder will burst. But a still more simple experiment, to shew the weight of the atmosphere, may be made in the following manner :-Fill a wineglass with water, and cut a piece of writing paper nearly to fit the top of the glass; lay the paper on the glass, which should be quite full; then, placing the palm of the hand over the paper, gently invert the glass and hold it bottom upwards. The palm of the hand may then be removed, and the paper will be pressed against the mouth of the glass by the atmosphere, so as to prevent the water from falling out, the pressure above being removed by the bottom of the glass. The use of the paper is to give a sufficiently dense medium to allow the air to press against the water.

"The atmospheric pressure on living bodies produces an effect which is rarely thought of, although of much importance, viz. keeping all the parts about the joints firmly together by an action similar to that

exerted on
on the Madgeburg hemispheres.*

The

broad surfaces of bones forming the knee-joint, for instance, even if not held together by ligaments, could not, while the capsule surrounding the joint remained air-tight, be separated by a force less than about a hundred pounds; but on air being admitted to the articular cavity, the bones at once fall to a certain distance apart. In the loose joint of the shoulder this support is of great consequence. When the shoulder or other joint is dislocated, there is no empty space left, as might be supposed, but the soft parts around are pressed in to fill up the natural place of the bone. When a thigh-bone is dislocated, the deep socket, called the acetabulum, instantly becomes like a cupping-glass, and is filled partly with fluid and partly with the soft solids. In all joints it is the atmospheric pressure which keeps the bones in such steady contact, that they work smoothly and without noise."-ARNOTT'S Physics, p. 349.

* The Madgeburg hemispheres here mentioned are merely two hollow iron cups, the edges of which are fitted so close that, when put together, they form an air-tight, hollow ball; the air being withdrawn from them by an air-pump, connected with the interior through the handles of the cups, they are pressed together by the external atmosphere, with a pressure of fifteen pounds on every inch of surface in the mouth of the cups. This experiment was the invention of Otto de Guericke, and was one of the first that drew attention to the material nature and properties of air. These hemispheres were considered so great a wonder, that it is recorded that the Emperor was present on the occasion of a public exhibition of them, when six of his coach-horses were required to pull them asunder.

The vent-peg, which we promised to explain in the last chapter, is another familiar illustration of this subject. When a cask is bunged close down, the pressure of air is removed from the surface of the liquid by the intervention of the cask, but when the tap is turned, to allow the beer to escape, the pressure is suffered to act on the lower surface of the beer in the cask, through the tap, and the consequence is, the beer will not run out till the vent-peg at the top is removed; this immediately restores the balance of pressure, and the weight of the beer causes it to escape.

The common pump, again, is a most useful application of this pressure to domestic purposes. As we have before stated, every inch of surface supports a weight of fifteen pounds. Now, if we can remove this fifteen pounds from one inch of the surface of the water in a well, whilst we allow the pressure to remain over the other part, it follows, that the water will be forced up in the part from which the pressure is removed; and if the water so forced up be enclosed in a tube, an inch square, we shall find that it will rise in this tube till the quantity contained in it will weigh fifteen pounds, which would require an altitude of thirty-four feet, the weight of water in the tube being equal to the weight of air on a similar surface of water in the well. The pump is an instrument for removing this pressure of the atmosphere from the