ment I cease the drawing in my breath, the water ceases to rise in my mouth,

Father. That is, when there is no longer a vacuum in the straw, the pressure within is just equal to that without, and consequently the water will rest at its natural level.

I will show you another striking instance of the effects of the air's pressure. This instrument (Plate v. Fig. 10.) is called the transferrer. The screw c fits on to the plate of the air-pump, and by means of the stop-cocks G and H, I can take away the air from both, or either of the receivers 1, K, at pleasure.

Emma. Is there a channel then running from c through D A B, and thence passing to x, and y?

Father. There is. I will screw the whole on the air-pump, and turn the cock G, so that there is now no communication from c to the internal part of the receiver I. At present you observe that

both the receivers are perfectly free. By turning the handle of the pump a few times the air is taken away from the receiver K, and to prevent its re-entrance I turn the stop-cock d. Try if you can

move it.

Charles. I cannot but the other is

loose.

Father.

The pressure of the atmosphere is evidently the same on the two receivers; but with regard to the glass I, the pressure within is equal to that without, and the glass is free: in the other, the pressure from within is taken away, and the glass is fixed. In this stage of the experiment you are satisfied that there is a vacuum in the receiver K. By turning the cock G, I open a communication between the two receivers, and you hear the air that was in I rush through the channel a B into K, Now try to move the glasses. Emma. They are both fixed: how is this?

Father. The air that was enclosed in the glass I is equally diffused between the two, consequently the internal pressure of neither is equal to the external, and therefore they are both fixed by the excess of the external pressure over the internal. In this case it could not be suction that fixed the glass 1, for it was free long after what might have been thought suction had ceased

to act.

Charles. What are these brass cups? (Plate v. Fig. 11.)

Father. They are called the hemisphe rical cups; I will bring the two, B, A, to gether, with a wet leather between them, and then screw them by D to the plate of the air-pump and having exhausted the air from the inside, I turn the stop-cock E, take them from the pump, and screw on the handle F. See if you two can sepa rate them.

Emma. We cannot stir them.

Father.

If the diameter of these cups

were four inches, the pressure to be over

E

come would be equal to 180tb. I will now hang them up in the receiver (Plate v. 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 vi. Fig. 13.) you may try very accurately to what weight the pressure of the atmosphere gainst the cups is equal.**

Emma. For when the weight w is carried ar enough to overcome pressure of the ups, it lifts up the top one.

Father. I have exhausted the air of this eceiver H, (Plate vi. Fig. 14.) consequently - is fixed down to the brass plate 1; to the late is joined a small tube with a stop

The principle of the steel-yard is explained, Vol. 1. Mechanics, Conversation XV.

CONVERSATION XXVIII

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 vi. 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?