correctly, he would say, he took away the external air from off the part of the body, and then the elastic force of the air within extends, and swells out the flesh ready for his lancets.

Emma. When I saw you cupped, he did not use an air-pump, but little glasses, to raise the flesh.

Father. Glasses closed at top are now generally made use of, in which the ope rator holds the flame of a lamp: by the heat of this the elasticity of the air in the glass is increased, and thereby a great part of it driven out. In this state the glass is put on the part to be cupped, and as the inward air cools, it contracts, and the glass adheres to the flesh by the difference of the pressures of the internal and exter nal air.

By some persons, however, the syringe is considered as the most effectual method of performing the operation, because by fame the air cannot be rarified more than

one half; whereas by the syringe a few strokes will nearly exhaust it.

Here is another little square bottle like that before mentioned, (Plate vi. Fig. 15.) only that it is full of air, and the mouth sealed so closely that none of it can escape. I enclose it within the wire cage B, and in this state bring them under the receiver, and exhaust the external air.

Charles. With what a loud report it has burst!

Father. You can easily conceive now in what manner this invisible fluid endea vours continually, by its elastic force, to dilate itself.

Emma. Why did you place the wire cage over the bottle?

Father. To prevent the pieces of the bottle from breaking the receiver, an accident that would be liable to happen without this precaution.

Take a new laid egg and make a small hole in the little end of it, then, with that end downwards, place it in an ale-glass unVOL. II.

Y

der the receiver, and exhaust the air; the whole contents of the egg will be forced out into the glass, by the elastic spring of the small bubble of air which is always to be found in the large end of a new-laid

CONVERSATION XXX.

The Compression of the Air.

FATHER. I have already alluded to the compressibility of air, which it is proper to describe here, it being a consequence of its elasticity for whatever is elastic, is capable of being forced into a smaller space. In this respect air differs very materially from other fluids.

Charles. You told us, that water was compressible in a very small degree.

Father. I did so; but the compression which can be effected with the greatest power, is so very small, that without the greatest attention and nicety in conducting

the experiments, it would never have been discovered. 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.

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Father. This bended tube A B C (Plate VI. Fig. 18.) 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 a b: now the air in each leg is of the same density, and, 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 will condense the air in the leg A B; for the air which filled the whole length of the leg is, by the weight of the quicksilver in c B, pressed into the smallest space a x, which space will be diminished as the weight is increased: so that by increasing the length