to be overcome would be equal to 180lbs. I will now hang them up in

the receiver (Plate 1, Fig. 12) and exhaust the air out of it, and you see they separate without the application of any force.

Charles. Now there is no pressure on the outside, and therefore the lower cup falls off by its own

gravity. Father.

With this steel-yard (Plate 11, Fig. 13) you may try very accurately to what weight the pressure of the atmosphere against the cups is equal*.

Emma. For when the weight w is carried far enough to overcome of the cups, it lifts up

the

pressure of the

the top one.

*The principle of the steel-yard is explained, Vol. I, of Mechanics, Conver. XV.

Father. I have exhausted the air of this receiver H (Plate 11, Fig. 14), consequently it is fixed down to the brass plate 1; to the plate is joined a small tube with a stop-cock x; by placing the lower end of the tube in a bason of water, and turning the cock, the pressure of the atmosphere on the water in the bason forces it through the tube in the form of a fountain. This is called the fountain in vacuo.

To this little square bottle A (Plate 11, Fig. 15) is cemented a screw valve, by which I can fix it on the plate of the air-pump, and exhaust its air; and you will see, that when there is no power within to support the pressure of the atmosphere from without, it will be broken into a thousand pieces.

Charles. Why did you not use a round phial?

Father. Because one of that shape would have sustained the pressure like an arch.

Emma. Is that the reason why the glass receivers are able to bear such a weight without breaking? Father. It is. If mercury be poured into a wooden cup c, made of willow, which is a very porous kind of wood (Plate 11, Fig. 16), and the air taken from under it, the mercury will, by the weight of the external air, be forced through the pores of the wood, and descend like a shower of rain.

CONVERSATION VI.

Of the Weight of Air.

EMMA. We have seen the surprising effects of the air's pressure; are there any means of obtaining the exact weight of air?

Father. If you do not require any very great nicety, the method is very simple.

This Florence flask (Plate 11, Fig. 17) is fitted up with a screw, and a fine oiled silk valve at D. I will now screw the flask on the plate of the air-pump, and exhaust the air. You

see, in its present exhausted state, it weighs 3 ounces and 5 grains. Charles. Cannot the air get through the silk?

Father. The silk, being varnished with a kind of oily substance, is impenetrable to air; and, being exhausted, the pressure upon the outside effectually prevents the entrance of the air by the edges of the silk; but if I lift it up by means of this sewingneedle, you will hear the air rush in. Emma. Is that hissing noise occasioned by the re-entrance of the air? Father. It is; and when that ceases, you may be sure the air within the bottle is of the same density as that without.

Charles. If I weigh it again, the difference between the weight now, and when you tried it before, is the