ing lighter than the beer, ascends to the top, by which a pressure is created without the assistance of the external air.

CONVERSATION IV.

Of the Lateral Pressure of Fluids. FATHER. It is time now to advance another step in this science, and to show that the lateral, or side pressure is equal to the perpendicular pressure.

Emma. If the upward pressure is equal to the downward, and the side pressure is also equal to it, then the pressure is equal in all directions.

Father. You are right. Though the side direction may be varied in many ways, yet there are only the upward, downward, and lateral directions. The two former we have shown are equal. That the side pressure is

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 state 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 I, for it was free long after what might have been thought suction had ceased to act.

Charles. What are these

Father. They are called the hemispherical cups; I will bring the two, BA, together, with wet leather between them, and then screw

F

them by D to the plate of the airpump 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 separate them.

Emma.

We cannot stir them.

Father. If the diameter of these cups were four inches, the pressure to be overcome would be equal to 180lb. I will now hang them up in

it gently down; the surface of the water in the tube is now even with that in the jar.

Emma. It is; and the bladder at the bottom is become flat.

Father. The perpendicular depths being equal, the pressure upwards is equal to that downwards, and the water in the tube is exactly balanced by the water in the jar. Let the tube be thrust deeper into the water.

Charles. Now the bladder is bent upwards.

Father. The upward pressure is estimated by the perpendicular depth of the water in the jar, measured from the surface to the bottom of the tube; but the pressure downwards must be estimated by the perpendicular height of the water in the tube, which being less than the former, the pressure upward in the same proportion over