of the column of mercury in C B, the air in the other leg will be more and more condensed. Hence we learn that the elastic spring of air is always, and under all circumstances, equal to the force which compresses it. Charles. How is that proved!

Father. If the spring with which the air endeavours to expand itself when it is compressed, were less than the compressing force, it must yield still farther to that force; that is, if the spring of the air in a x were less than equal to the weight of the mercury in the other leg, it would be 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 higher.

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

condensing syringe and a squirt is, that, in the former, there is a valve that opens downwards, by which air may be forced through it, but the instant that the downward pressure ceases, the valve, by means of a strong spring, shuts of itself, so that none can re

turn.

Emma. Will not air escape back, during the time you are forcing in more of the external air?

Father. That would be the case if the syringe pipe went no lower than that part of the vessel which contains the air, but it reaches to a considerable depth in the water, and as it cannot find its way back up the pipe, it must ascend through the water, and cause that pressure upon it which has been described.

Charles. To what extent can air be compressed?

Father. If the apparatus be strong enough, and a sufficient power applied, it may be condensed several thousand times; that is, a vessel which will contain a gallon of air in its

natural state may be made to contain several thousand gallons.

By means of a fountain of this kind, young people, like yourselves, may receive much entertainment with only a few additional jets, which are made to screw on and off. One kind is so formed that it will throw up and sustain on the stream a little cork ball, scattering the water all round. Another is made in the form of a globe, pierced with a great number of holes, all tending to the centre, exhibiting a very pleasing sphere of water. One is contrived to show, in a neat manner, the composition and resolution of forces explained in our first volume.* Some will form cascades ; and by others you may, when the sun shines at a certain height in the heavens, exhibit artificial rainbows.t

We will now force in a fresh supply of air, and try some of these jets.

• See Vol. I. Of Mechanics. Conversation XIII. †This phenomenon is described and explained in Conversation XVIII.

Emma. I observed in the upright jeß, that the height to which the water was thrown was continually diminishing.

Father. The reason is this; that in proportion as the quantity of water in the fountain is lessened, the air has more room to expand, the compression is diminished, and consequently the pressure becomes less. till at length it is no greater within than it is without, and then the fountain ceases altogether.

CONVERSATION XXXI.

Miscellaneous Experiments on the Air-Pump.

FATHER, I shall, to-day, exhibit a Few experiments, without any regard to the particular subjects under which they might be arranged.

In this jar of water I plunge some piees of iron, zinc, stone, &c. and you will see that when I exhaust the external air, y bringing the jar under the receiver of the air-pump, the elastic spring of air contained in the pores of these solid substances will force them out in a multitude of globules, and exhibit a very pleasing specacle, like the pearly dew-drops on the