英文科學讀本 第四冊·Lesson 26 More about the Pressure of Liquids

Lesson 26 More about the Pressure of Liquids

We are now in a position to inquire a little further into the subject of liquid pressure, said Mr. Wilson. "You remember that, in one of our recent lessons, we proved, by pouring water into a tin box, that liquids press downwards, and the more water we put in the box the heavier it became. This is just what we might expect, and it proves that the greater the depth of water, the greater the pressure downwards.

Suppose we have another experiment. I have here a long, round, cardboard case, which I have plugged up at one end, so as to make it watertight. Fred shall come and hold it while I fill it with water. If I bore a hole in the side, the water will flow out. Now notice; I will bore several holes, one below the other. The water flows out through each. But are all the streams alike? Look! the top stream flows quite gently compared with the rush of the bottom one, and each one, from the top downwards, flows with greater force than that above it.

Now what does this prove?

It proves that the pressure of the water sideways increases with the depth, sir.

Quite true, Fred. It is for this reason that, in constructing canals, reservoirs, and embankments, the walls or banks are made thicker at the bottom than the top. The greater the depth of water, the greater the pressure sideways. Here I have the glass cylinder and disc, which we used before, in one of our experiments. When I lower the cylinder and disc into the water and let go the string, what happens?"

The disc remains close to the bottom of the cylinder, kept there by the upward pressure of the water, sir.

Exactly. Now I will pour water gently into the cylinder. This water does not run out at the bottom, you sea. It gradually rises in the cylinder. What does that prove?

It proves that the upward pressure of the water against the disc is greater than the weight of the water in the cylinder above it.

You are quite right, my lad. Suppose, now, we pour some more water into the cylinder. The water continues to rise in it till at last the disc falls away. It required the downward pressure of all that water in the cylinder to overcome the upward pressure on the disc. Now lower the cylinder and disc again, but to a greater depth, and I will pour in water as before. This time more water is required in the cylinder to detach the disc. What does that prove?"

It shows plainly, sir, that the upward pressure of the water is greater than it was before; it requires a greater weight of water pressing downwards in the cylinder to overcome the upward pressure on the disc.

Just so, said Mr. Wilson; "and now I think we clearly understand that the greater the depth, the greater the pressure downwards, upwards, and sideways. In other words, the pressure in all directions increases with the depth.

Now before we leave the cylinder and disc, I want to show you another little experiment. We will lower them as before into the water and let go the string. The disc remains close to the cylinder. Notice that I may move it about wherever I please, horizontally, in the water, and the disc does not fall away. The pressure upward is always the same at the same depth.

Now I will raise it a little in the water, and the disc falls away, because the upward pressure is not so great at that depth as it was when lower in the water. Let us return to our cardboard case. I will first plug up all the holes, and then pierce it with a circle of holes at the same level. If we now fill it with water, we shall see that equal streams flow from all the holes with equal force.

This means that, at the same depth, the pressure downwards and sideways is equal. Our experiments therefore prove that, at the same depth, water, and, of course, all liquids press equally in every direction.