英文科学读本 第六册·Lesson 50 Water as a Machine

Lesson 50 Water as a Machine

We have lately been discussing certain contrivances which we call machines. Each of these machines is employed by man for the purpose of turning to the best account the various natural forces. You remember, of course, that we defined a machine to be any contrivance for transmitting a force from one point to another, or for altering the direction or intensity of the force. Now, I want to introduce to you another machine, and a very important one—I mean water. As we are going to think of water as a machine, it is clear that it must possess certain properties peculiar to itself, which enable it, like other machines, to accomplish certain work. To be a machine, as we now understand machines, it should be able to transmit a force from one point to another; it should be able to alter the direction of the force; it should be able to increase the intensity or magnitude of the force. Water can do all these things, and it does them because of two properties which it possesses in common with all liquids.

One of the important discoveries which our earlier lessons led us to make with regard to water is that it is incompressible. You no doubt remember our experiment with the pop-gun and the squirt. We filled each of these with water, and, after closing the hole firmly with the finger, tried to force the piston down. With all our trying it refused to move, because water will not be compressed, or squeezed into smaller space. I want you to keep this fact well before you now. If a vessel of any kind is filled with water, no amount of pressure will force that water into a smaller space. Another important truth which we learned in those early lessons was that water presses equally in all directions. I remember I showed you this by means of a hollow india-rubber ball, which I first pierced all over with small holes, and then filled with water. When I squeezed the ball, jets of water were forced out on all sides.

Suppose we repeat the experiment with this wooden pop -gun. I have already drilled some small holes all round the end of it, and I will now fill it with water. I have only to press the end firmly against the palm of one hand, and force the piston down with the other, and a jet of water will rush out through every hole. The pressure applied to the piston rod in one direction has been transmitted by the water in every direction. It is because water is incompressible, and because it transmits pressure equally in all directions, that we are able to use it as a machine. But to be an effective machine it must also alter the intensity or magnitude of the force. Let us see whether it does this.

Imagine, if you can, two tubes, one small, the other very large, with a connecting pipe between them. If we poured water into either one of those tubes, you know that it would flow into the other, and the level in both would be the same, because water always finds its own level. Imagine each tube provided with a close-fitting piston, resting on the top of the water; let the under surface of the small piston be exactly 1 square inch, and that of the other 100 square inches. Now, if liquids transmit pressure equally in all directions, it is clear that whatever pressure was applied to the small piston would be transmitted by the water to every square inch of the surface of the larger piston. That is to say, if a 1 lb. weight were placed on the small piston, we should expect to find that every square inch of the surface of the large piston resting on the water would be pressed upwards with a force of 1 lb.; the entire upward pressure on the piston being 100 lbs.

Now, as a matter of fact, if we had two such piston-fitted cylinders, a weight of 1 lb. on the small piston could only be balanced by 100 lbs. placed on the large piston. This tells us that the water on the under side of the piston is pressing upwards with a force of 100 lbs., and it derived that pressure from the 1 lb. weight placed on the small piston. Hence we see that water is actually a machine. It is able to transmit a force from one point to another; it is able to alter the direction of the force; it is able to increase the intensity or magnitude of the force."