Lesson 24 Pressure of Liquids

Fred and Willie, I shall want you to come to the front this morning, said Mr. Wilson, "and help me with one or two experiments. You shall begin, Fred, by holding this biscuit tin in your hands, while Will places in it some of these bricks, one by one.

What do you notice, Fred?

The box is getting heavier than it was, sir, said Fred, "and every brick adds to the weight."

What do you mean by weight?

I mean the downward pressure.

Quite right, Fred; the top brick presses upon the one below, and this transmits the pressure to the bottom of the box. The whole weight is pressing on the bottom; the bricks do not even touch the sides of the box. Now we will remove the bricks and pour water into the box.

What do you notice now as the water rises?

The downward pressure increases, sir, just as it did when we put the bricks into the box.

Yes, the upper layers of water (like the upper bricks) press upon those below, and they transmit the pressure through to the bottom of the box. Liquids press downwards. Now stand the box on the table, and I will bore a hole in the side of it. The water, you see, runs out in a stream. Let us plug up this hole and make another on the opposite side. See, the water flows out through this hole too. We will plug up this, and I will bore a hole in each of the other sides. Now watch what happens when I remove the plugs. The water streams out sideways in four different directions.

What does this prove?

It proves that water presses sideways as well as downwards.

Right. The upper layers of water (like the upper bricks) press on those below. The bricks transmit their pressure in one direction only—downwards. The water, on the other hand, transmits its pressure not only downwards but sideways as well. Now, Willie, you shall lay this flat piece of wood on the water in the bowl, and try to force it down. What do you notice?"

It is not easy to force it down, sir; something seems to be trying to force it up.

You are quite right, my boy; the water presses upwards as well as in other directions, and it is forcing the wood up. Let us try something else," said Mr. Wilson. "I have here a glass cylinder, with a piece of bladder tied over it at one end, so as to hang from it like a bag. You shall plunge this into the water. What do you see?"

The bladder is forced upwards inside the cylinder, sir, as I push it down into the water.

What forces it up?

I suppose it is the water pressing upwards, sir, as before.

Right, again, my lad. Now we will remove the bladder from the end of the cylinder, and cover it instead with this thin disc of tin. There is a piece of fine cord attached to the center of the disc, and this we will pass up through the cylinder. I keep the disc close against the bottom of the cylinder while I lower it into the water, and then I let go the string. Notice what happens. Does the disc fall away?

No, sir; it remains close to the bottom of the cylinder, as it was at first. It must be kept there by the upward pressure of the water.

Quite right, said Mr. Wilson, "it is the upward pressure of the water that does it, and now we understand that liquids press downwards, sideways, upwards—in fact in all directions."