J. A. COOLIDGE

That bodies heavier than water sink, and if lighter than water float, is a fact so well known as to make any such statement unnecessary. Yet the relation these bodies bear to water and other liquids and the cause of a body's sinking in one liquid and not in another afford us opportunity for valuable experiment and additional knowledge.

If a stone or other body falls through the air or sinks in water, it is because a downward force called gravity pulls it, and every hody will move downward, i. e., towards the earth's centre, until something prevents further motion or, better, until the force pushing upward balances the force downward. The downward force we call the weight of the body. In every floating body we call the force holding it up, the buoyancy. A few experiments will teach us some interesting facts about buoyancy.

Let us make first two blocks 2x3x3 in. and a little box of 1/4 in. stock that will just hold one of these blocks. See Fig. 23. The box and the block may be of pine and should be coated thoroughly with melted paraffine, which may be obtained from a common candle. The wax will make them waterproof. A large glass jar 10 in. deep and 6 in. in diameter will be a great convenience, although a deep pan or pail will answer.

Experiment XIX.

Take the box and block to a grocer's or provision store and weigh them as accurately as the scales will allow. If you know some druggist who would help you weigh them to 1-10 oz., the experiment would be more successful. Fill the box with cool water and weigh again. The box and the blocks, if accurate in measurement, contain just 18 cubic inches in volume. By dividing the weight of the block by 18 you will have the weight of 1 cubic inch of wood. This will be the density of this wood. From the weight of the box filled with water subtract the weight of the empty box and divide this fig"ure by 18 and you will find the density of water, which is about .58 of an ounce per cubic inch. Commonly 62.5 pounds is allowed as the weight of a cubic foot of water.

Experiment XX.

Float the box in the jar of water and measure carefully the depth that it sinks in water. If one corner sinks more than another, all four corners should be measured. Calculate the volume, ABCD, under water. See Fig. 24. This will be the amount of water displaced. Find the weight of this water, allowing .58 oz. for every cubic inch. Does it not equal the weight of the box ? It should, for every floating body displaces its own weight.

Experiment XXI. Take a tin vegetable can, measure its diameter. Calculate the size of the bottom by multiplying radius x radius x 3 1-7. Put some sand in the bottom and make it float vertically. Measure the depth under water.

The volume of water displaced will be the area of the bottom multiplied by the depth under water. Find the weight of this number of cubic inches of water by multiplying this volume by .58 oz. Weigh the can with the sand in it. Of course you know that the hollow can floats because, although of heavier material than water, it displaces its own weight, and by sinking deeper in the water it would still float and hold more sand inside. See Fig. 25.

These two experiments show how a wooden ship, and even one made of steel and iron, floats while holding an immense cargo. A vessel made of iron or steel, although of material seven times as heavy as water, has large airtight compartments , so that buoyancy is sufficient to float ship, passengers, cargo, and many other things. I have in my hands the description of one of the great ocean steamers, not the largest, yet in itself a floating city. Try with me to get some conception of her size. She is 580 ft. long. Pace off 200 of your own steps and see how far you have gone and you will have some idea of the fength. It would take you two minutes to walk from bow to stern. In a town or city where lots have 50 feet frontage this vessel would, if placed in the street, pass by eleven houses. She is 62 feet wide and would fill many an ordinary street from side to side, touching the houses on both sides. She is 46 feet deep. We could stand in the third story window of our houses and look up to the deck of this boat, or step from the deck to the roof of a five storied building. She weighs 13,000 tons empty, and will carry 12,000 tons of cargo. She carries about 800 single loads of coal just for her own use and, although floating in an ocean of water, carries over 2000 tons of fresh water on board. Over 1,000 people find every accommodation for a long cruise, as many as are found in many small towns, and all are comfortably housed and fed. Every modern convenience, electric lights, baths, ice, fire alarm and apparatus is found on board. She makes her own ice and carries apparatus for evaporating fresh water from salt, enough to make 40 tons every day. Immense engines and shafting over 200 feet long and 16 inches in diameter, of 9,000 horse power, drive this immense floating palace through the water. Can you not see how important it is to know about the buoyant effect of water in order to provide for floating such a vessel ?

The effect of salt water as compared with fresh in its buoyancy, can be studied with profit.

Experiment XXII.

Take two jars or pails. Fill them both with fresh water. Now take an egg, hold it in the water and let go. It will sink. In one of the jars pat a handful of salt and stir. If the egg still remains on the bottom, p.ut in more salt. While this experiment is going on take one block and float it in fresh water, then in salt. In each case notice the depth to which it sinks. With a good deal of salt in the water the egg may be made to float, and the block floats with much less under water than before. In an ocean of salt water a vessel sinks less deep than in fresh because the displaced water is heavier and, therefore, the buoyancy is greater. We see, then, that heavy liquids are more buoyant than lighter ones. In mercury, the heaviest liquid, solid iron, or even lead, will float readily.