is more or less necessary if
successful boats are to be made. Aside from this, they are terms that
every boy who claims an interest in boats should understand.

"How does a steel boat float?" is a question that many boys ask. The
reason they usually designate a steel boat is probably because steel is
so much heavier than water. But many things heavier than water can be
made to float if they are in the form of a boat. Concrete, for instance,
is now being used in ship construction, and this substance, when
reinforced with steel rods, is very much heavier than water.

Before learning how a boat floats, what is known as "specific gravity"
must be thoroughly understood. Gravity is a force that is continuously
"pulling" everything toward the center of the earth. It is gravity that
gives a body "weight." Some substances are heavier than others; or, to
be more correct, it is said that the specific gravity of one substance
is greater than that of another. It will be well to keep in mind that
specific gravity merely refers to weight. It is simply a scientific
term. The specific gravity of a substance is always expressed by a
figure that tells how much heavier any substance is than water, because
water has been chosen as a standard.

The specific gravity of water is 1. The specific gravity of gold is
19.26, meaning that it is about 19-1/4 times heavier than water. The
specific gravity of a piece of oak is 0.86, which shows that it is not
quite so heavy as water. One cubic foot of water weighs 62.42 pounds.
It will be understood that a cubic foot of gold would weight 19.26 x
62.42, because it is 19.26 times heavier than water. A cubic foot of
oak, however, would weigh only 54 pounds, because it has been found that
it has a specific gravity of only 0.86 which is less than water.

[Illustration: FIG. 1]

A cubic foot of oak (see Fig. 1), with a weight of 54 pounds, will float
when placed in water. The cubic foot of brass (_B_), however, will not
float, because it weights 8.1 times as much as water. For the present,
then, it can be said that a substance lighter than water will float in
water, but that substances heavier than water, such as iron, lead, gold,
silver, etc., will not float. If the cubic foot of oak (_A_) were
placed in water, it would sink to the depth shown at _C_. When the block
sinks into the water, a certain amount of water will be forced away or
"displaced"; that is, the block in sinking occupies a space that wa