Читать книгу Common Science - Carleton Washburne - Страница 16

Table of Contents

Why iron ships float. When people first talked about building iron ships, others laughed at them. "Iron sinks," they said, "and your boats will go to the bottom of the sea." If the boats were solid iron this would be true, for iron is certainly much heavier than water. But if the iron is bent up at the edges—as it is in a dish pan—it has to push much more water aside before it goes under than it would if it were flattened out. The water displaced, or pushed aside, would have to take up as much room as was taken up by the pan and all the empty space inside of it, before the edge would go under. Naturally this amount of water would weigh a great deal more than the empty pan.

But suppose you should fill the dish pan with water, or suppose it leaked full. Then you would have the weight of all the water in it added to the weight of the pan, and that would be heavy enough to push aside the water in which it was floating and let the pan sink. This is why a ship sometimes sinks when it springs a leak.

You may be able to see more clearly why an iron ship floats by this example: Suppose your iron ship weighs 6000 tons and that the cargo and crew weigh another 1000 tons. The whole thing, then, weighs 7000 tons. Now that ship is a big, bulky affair and takes up more space than 7000 tons of water does. As it settles into the water it pushes a great deal of water out of the way, and after it sinks a certain distance it has pushed 7000 tons of water out of the way. Since the ship weighs only 7000 tons, it evidently cannot push aside more than that weight of water; so part of the ship stays above the water, and all there is left for it to do is to float. If the ship should freeze solid in the water where it floated and then could be lifted out of the ice by a huge derrick, you would find that you could pour exactly 7000 tons of water into the hole where the ship had been.

But if you built your ship with so little air space in it that it took less room than 7000 tons of water takes, it could go clear under the water without pushing 7000 tons of water aside. Therefore a ship of this kind would sink.

The earth's gravity is pulling on the ship and on the water. If the ship has displaced (pushed aside) its own weight of water, gravity is pulling down on the water as hard as it is on the ship; so the ship cannot push any more water aside, and if there is enough air space in it, the ship floats.

Perhaps the easiest way to say it is like this: Anything that is lighter than the same volume of water will float; since a cubic foot of wood weighs less than a cubic foot of water, the wood will float; since a quart of oil is lighter than a quart of water, the oil will float; since a pint of cream is lighter than a pint of milk, the cream will rise. In the same way, anything that is lighter than the same volume of air will be pushed up by the air. When a balloon with its passengers weighs less than the amount of air that it takes the place of at any one time, it will go up. Since a quart of warm air weighs less than a quart of cold air, the warm air will rise.

You can see how a heavy substance like water pushes a lighter one, like oil, up out of its way, in the following experiment:

Experiment 11. Fill one test tube to the brim with kerosene slightly colored with a little iodine. Fill another test tube to the brim with water, colored with a little blueing. Put a small square of cardboard over the test tube of water, hold it in place, and turn the test tube upside down. You can let go of the cardboard now, as the air pressure will hold it up. Put the mouth of the test tube of water exactly over the mouth of the test tube of kerosene. Pull the cardboard out from between the two tubes, or have some one else do this while you hold the two tubes mouth to mouth. If you are careful, you will not spill a drop. If nothing happens when the cardboard is pulled away, gently rock the two tubes, holding their mouths tightly together.