met and divided the air by the edge only continued to move the longest, while they were both exposed to it; but when that is removed, they both stop together, because there is nothing now to retard their motion, but the friction on the pivots, which is the same in both cases. Take this guinea and a feather: let them both drop from your hand at the same instant.

Charles. The guinea is soon at rest at my feet, but the feather continues floating about. Is the feather specifically lighter than air?

Father. No: for if it were, it would ascend till it found the air no heavier than itself; whereas, in a minute or two, you will see the feather on the floor as well as the guinea: it is however so light, and presents so

large a surface to the air, in comparison to its weight, that it is considerably longer in falling to the ground than heavier bodies, such as a guinea. Take away the resisting medium, and they will both reach the bottom at once.

Emma. How will you do that? Father. Upon this brass flap (Plate 1, Fig. 3) I place the guinea and the feather, and having turned up the flap, and shut it into a small notch, I fix the whole on a tall receiver, with a piece of wet leather between the receiver and brass. will now exhaust the air from under the receiver, by placing it over the air-pump, and, if I turn the wire f a little, the flap will slip down, and guinea and feather will fall with equal velocities:


In perfect void

All substances with like velocity

Descend, nor the soft down outstrips the gold.


Charles. They are both at the bottom, but I did not see them fall.

Father. While I repeat the experiment, you must look stedfastly to the bottom, because the distance is too small for you to trace their motion; but, by keeping your eye at the bottom, you will see the feather and guinea arrive at the same instant.

In this glass tube (Plate 1, Fig. 4) is some water, but the air is taken away, and the glass completely closed. Turn it up quick, so that the water may fall on the other end. Emma. It makes a noise like the stroke of a hammer,

Father. And for that reason,it is

usually called the philosophical hammer. The noise is occasioned through the want of air to break the fall: for if I take another glass, in all respects like it, but having the air enclosed in it, as well as water, you may turn it as often as you please with hardly any noise.

Charles. Perhaps the air breaks the fall of the water by dividing its particles.

Father. It acts, with respect to water, as water acts with regard to the fall of any other substance thrown into it: it impedes the velocity of the falling body.


Of the Torricellian Experiment.

CHARLES. If, by means of the air-pump, you cannot perfectly exhaust the air from any vessel, by what means is it done?

Father. This glass tube is about 36 inches long, and open at one end only. I fill it very accurately with quicksilver, and, placing my thumb over the open end, I invert the tube, and plunge it into a vessel of the same metal, taking care not to remove my thumb till the end of the

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