THE WEIGHT OF AIR.

53

Charles. Do you mean then, that after the first turn of the handle, the air in the bottle is twice as rare as it was at first; and after the second, third, and fourth turns, it is four times, eight times, and sixteen times as rare as it was when you began ?

Father. That is what I meant; carry on your multiplication, and you will find that after the twelfth turn it is 4096 times more rare than it was at first.

Emma. I now understand that, though absolute exactness be not attainable, yet, in weighing this quart of air, the error is only equal to the 4096th part of the whole, which quantity may, in reasoning on the subject, be overlooked.

Father. I will exhaust the flask again of its air, and, putting the neck

F 3

of it under water, I will lift up the silk valve and fill it with water. Now dry the outside very thoroughly and weigh it.

Charles. It weighs 27 ounces.

Father. Subtract the weight of the flask, and reduce the remainder into grains, and divide by 144, and you will obtain the specific gravity of water compared with that of air.

Charles. I have done it, and the water is something more than 800 times heavier than air.

Father. Since, then, the specific gravity of water is always put at 1, that of air must be asth, at least according to this calculation; but following the more accurate experiments of Mr. Cavendish and others, whose authority may be safely appealed to, the specific gravity of air is 800 times

SPECIFIC GRAVITY OF air.

55

less than that of water, when the barometer stands as high as 30 inches.

Can you tell me what the air in

this room weighs? the length of the room is 25 feet, the height 1C, and the width 124?

Emma. I multiply these three numbers together, and the answer is 3281.25; or the room contains a little more than 3281 cubic feet: now a cubic foot of water weighs 1000 ounces, therefore the weight of the roomfull of water would be 3,281,000 ounces; but air being 800 times lighter than water, the air in the room will weigh 3,281,000+ 800 = 4101/ ounces = 256 lbs. 5 oz. It seems, however, surprising that the air, which is invisible, should weigh so much.

56

PNEUMATICS.

CONVERSATION VII.

Of the Elasticity of Air.

FATHER. I have told you that air is an elastic fluid. Now it is the nature of all elastic bodies to yield to pressure, and to endeavour to regain their former figure as soon as the pressure is taken off. In projecting an arrow from your bow, you exert your strength to bring the two ends of the bow near together, but the moment you let go the string, it recovers its former shape: the power by which this is effected is called elasticity.

Emma. Is it not by this power that India-rubber, after it has been stretched, recovers its usual size and form?

Father. It is: and almost every thing that you make use of possess this property in a greater or less degree: balls, marbles, the chords of musical instruments, are all elastic.

Charles. I understand how all these things are elastic: but do not see in what manner you can prove the elasticity of the air*.

Father. Here is a bladder, which we will fill with air, and tie up its mouth to prevent it escaping again. If you now press upon it with your hand, its figure will be changed; but the moment the pressure is removed it recovers its round shape.

* See Vol. I, of Mechanics, Conver. XIII.