127. =Upward Pressure as the Depth.=--It has been shown that the downward and the lateral pressures are as the depth. The same is true of the upward pressure, for it is produced by the same cause--the attraction of the earth. Let us look at this. Why is any particle of a fluid pressed upward at all? It is from the struggle on the part of the neighboring particles to get below it. And why this struggle? It is from the attraction of gravitation, and so the greater this attraction the greater is the upward as well as the downward pressure. The upward pressure therefore differs at different depths as the downward pressure does. Thus, in Fig. 81, the upward pressure against the layer or stratum of particles, _b_, is greater than that against _a_, for the same reason that the downward pressure on _b_ is greater than that on _a_. But the two pressures at _b_ are equal, and so are they at _a_, and therefore each stratum remains at rest. [Illustration: Fig. 82] 128. =Experiments.=--Some very neat experiments can be tried, showing that the upward pressure varies with the depth. Take a large glass tube, A B C D, Fig. 82, and let there be fitted to one end a circular plate of brass, which may be held there by a string, F. Thus arranged, plunge it quite deep into water, and you will find that you will not need to hold on to the string, for the brass disk will be held tight to the tube by the upward pressure of the water. Now draw up the tube slowly, and at length the disk will fall from the end of the tube. Why? Because the end of the tube has come to a point where the upward pressure of the water is less than the downward pressure of the disk. To have this experiment succeed, the end of the tube where the disk is applied must be very even and smooth. Another experiment may be tried in this way. Tie to one end of a glass tube a piece of thin India-rubber or bladder, and fill the tube partly with water. The India-rubber will of course bulge out or be convex from the weight of the water. Press the closed end down a little way in a vessel of water, so that the level in the tube shall be above the level in the vessel. The India-rubber is still somewhat convex, because, as the upward pressure upon it is in proportion to its distance from the surface of the water outside of the tube, it is not as great as the pressure downward of the higher water in the tube. Push the tube now so far down that the level in the tube is the same with that in the vessel. The India-rubber is now flat, because the downward and upward pressures upon it are equal, just as would be the case with a stratum of water in place of it. But press the tube lower down, and the India-rubber bulges upward into the tube, because the upward pressure is now greater than the downward.