but which in their natural state would take up too much space. The same property is also applied to the proof of cables, by tearing them; and to the pulling up of trees.

ments it would never have been dis

covered. Air, however, is capable of being compressed into a very small space compared with what it naturally possesses.

Emma. The experiment you made by plunging an ale-glass with its mouth downwards, clearly proved that the air which it contained was capable of being reduced into a smaller space.

Father. This bended tube A B C is closed at A and open at C. It is in the common state, full of air. I first pour into it a little quicksilver, just sufficient to cover the bottom ab now the air in

L

B

each leg is of the same density, and as as that contained in A B cannot escape, because the lighter fluid will be always uppermost, when I pour more quicksilver in at c, its weight

in the next seven times 16 feet, and so on, continually increasing according to the same law.

Father. How many feet has it fallen altogether at the end of the third second?

Emma. I recollect this very well; the whole space through which it will fall in three seconds is nine times 16, or 144 feet; because the rule is, that the whole spaces described by falling bodies are in proportion to the squares of the times, and the square of three is nine, therefore, if it fall through 16 feet in the first second, it will in three seconds fall through nine times 16, and in five or eight seconds it will descend in the former case through 25 times 16 feet, and in the latter through 64 times 16 feet, for 25 is the square of five, and 64 is the square of eight. The example of the

forced into a yet smaller space; but if the spring were greater than the weight pressing upon it, it would not have yielded so much; for you are well aware that action and re-action are equal, and act in opposite directions.

You can now easily understand why the lower regions of the atmosphere are more dense than those which are higher.

Emma. Because they are pressed upon by all the air that is above them, and therefore condensed into a

smaller space.

Father.

Consequently the air becomes gradually thinner or rarer, till, at a considerable height, it may be conceived to degenerate to nothing. The different densities of the air may be illustrated by conceiving twenty or thirty equal fleeces of wool placed one

upon another; the lowest will be forced into a less space, that is, its parts will be brought nearer together and it will be more dense than the next; and that will be more dense than the third from the bottom, and so on till you come to the uppermost, which sustains no other pressure than that occasioned by the weight of the incumbent air.

Let us now see the effects of condensed air, by means of an artificial fountain. This vessel is made of strong copper, and is about half full of water.. With a syringe that screws to the pipe BA I force a considerable quantity of air into the vessel, so that it is very much condensed. By turn

ing the stop-cock B while I take off the