英文科学读本 第四册·Lesson 30 Floating bodies

Lesson 30 Floating bodies

Our last experimental lesson, said Mr. Wilson, "taught us that when we immerse a solid body in the water, the water, by its upward pressure, tries to bear or buoy it up. Certain bodies, it is true, sink in the water when unsupported, but this is because they are heavier than the water. The same bodies would refuse to sink if they were placed in a heavier liquid, say mercury. Here is some mercury in this bowl. Pieces of stone, tin, iron, silver, copper, and lead all rest on the surface of this liquid.

You shall come to the front and help me again, Fred. I want you to place this piece of cork and these pieces of wood in the bowl of water. They rest on the surface of the water. Now try and force them down to the bottom. They will not remain there; they immediately rise to the top again.

Here is a bladder filled with air. It rests lightly on the water. Try and force it down. It springs upwards to the surface again. The upward pressure of the water forces all these things up. It tried to do the same with the piece of glass the other day, but the glass was too heavy. Its weight overcame the upward pressure of the water, so that it sank.

We call this upward pressure of water its buoyancy, that is, its power of buoying bodies up, so as to make them rest on its surface. Bodies which rest on the surface of a liquid in this way are said to float. But why do they float?

They float because the upward pressure of the liquid is greater than their downward pressure or weight, sir.

Quite right, Fred. Now I want you to notice the way in which the various bodies float. The bladder rests lightly on the water, the cork sinks into it a little way, the pine, beech, and oak woods sink lower down. Let us see what all this means. I have here a small tin box, in one side of which I have bored a hole. The box is filled with water up to the hole. I will suspend the piece of glass from the balance, and let it dip into the water just as we did before. Note carefully what happens. You know that the glass loses one ounce in weight when it dips in the water. What do you see now?"

Some of the water is running out of the hole into the basin below, sir.

So it is. But why should it run out? I will tell you. The water and the glass could not occupy the same space, so some of the water runs out at the hole as the glass dips down, to make room for it. The piece of glass forces out exactly its own bulk of water. If this water were collected and weighed, it would be found to weigh just one ounce—exactly what the glass lost in weight. That is to say, the upward pressure of the water is equal to the actual weight of the quantity of water which the body displaces. If the body were equal in weight to its own bulk of water, it would float with its upper surface on a level with the surface of the water. It would, in fact, displace a quantity of water equal to itself, bulk for bulk.

You know that a piece of iron sinks in the water, because it weighs much more than its own bulk of that liquid. But I put this canister in the water, and it floats. Why is this? The canister, although made of heavy metal, is hollow and filled with air only. It weighs much less than its own bulk of water, and thus the water can buoy it up. This explains why our great iron ships float on the sea. They are lighter than the water, bulk for bulk, and the buoyancy of the water makes them float."