We are in the habit of using the words =heavy= and =light= rather
carelessly. We call things that are easily lifted light, and things that
are hard to lift heavy. We say that sand, which is blown about by the
wind, is light, and that a block of wood is heavy, and yet we have just
seen that sand is heavier, bulk for bulk, than wood. In order to get rid
of this double meaning, the weight of a volume of any liquid or solid,
in proportion to the weight of the same volume of water at a known
temperature and pressure, is called its =specific gravity=. Water being
taken as 1, anything a volume of which is twice as heavy as the same
volume of water is said to have the specific gravity 2; if three times,
3; if four and a half times, 4·5, and so on. Thus the specific gravity
of any liquid or solid expresses its density in proportion to that of
water under the same conditions. Sawdust, oil, and spirit have a less
specific gravity than water, while treacle, sand, and quicksilver have a
greater specific gravity. In this sense, the former three substances are
=light=, while the latter three are =heavy=.
25. =Things of greater Specific Gravity than Water sink in Water; Things
of less Specific Gravity float.=
Here are two tumblers of water. Throw some sand into one and some
sawdust into the other. What happens? The sand sinks to the bottom, the
sawdust floats at the top. We may stir them up as we like, but the sand
will tumble to the bottom and the sawdust, as obstinately, rise to the
top. Thus that which is lighter than the water floats, and that which is
heavier (bulk for bulk) sinks. So, if we pour some oil into the water,
it floats, and if we pour some coloured spirit in carefully, it also
floats; while treacle and quicksilver sink to the bottom, just as the
iron-filings do.
We saw that the iron-filings sank, because iron is heavier than water.
Here is a piece of the thin tinned sheet-iron that they make tin boxes
of. What will happen if we drop it into the water? It is heavier than
water, bulk for bulk, and therefore it will sink as you see it does.
But now here is a “tin” canister made of this very same tinned
sheet-iron. We drop that into the water, and you see it does not sink at
all, but floats at the top as if it were made of cork. Here is a
perplexity. We were sure just now that iron is heavier than water, and
here is an iron box floating! Is this an exception to the law? Not at
all; for what we said was that a thing would float if it were lighter,
bulk for bulk, than water. Now let us weigh the tin box, and having
weighed it let us next try to find out how much the same bulk of water
weighs. This may be done very simply, for the walls of the box are very
thin, so that the inside of the box is very nearly as large as the whole
box. Consequently, if we fill the box with water, and then weigh the
water, we shall find out, very nearly, what is the weight of a bulk of
water as great as that of the box. But if we do this, we shall find that
the water which was contained in the box, weighs very much more than the
box does. So that, bulk for bulk, the box, although it is made of iron,
is really lighter than water, and that is why it floats.
You will all have heard of the iron ships which are now so common, and
you may have wondered how it is, that ships made of thick plates of iron
riveted together, and weighing many thousand tons, do not go to the
bottom. But they are nothing but our tin canisters on a great scale, and
they float because each ship weighs less than a quantity of water of the
same bulk does.
It is because of this property of water to bear up things lighter than
itself, and because of that other property of being easily moved which
the particles of water have, that the sea, and rivers, and canals, are
such great highways for mankind.
For there is nothing so heavy that it may not be made to float in water,
if the box which holds it is large enough to make the weight of the
whole less than the weight of the same bulk of water. And then, having
once got the weight to float, the particles of water are so easily
moved, that the force of the winds, or of oars, or of paddles, readily
causes it to slip through the water from one place to another.
26. =A Body which Floats in Water always occupies as much Space beneath
the level of the Surface of the Water as is equal to the Volume of Water
which weighs as much as that Body; in other words, it displaces its own
Weight of Water.=
A cubic inch of water weighs about 252 grains and a half. Suppose that
the tin box in the previous experiment was square, and had the bulk of
100 cubic inches, then the weight of a corresponding volume of water
would be 25,250 grains. If the box weighed 8,416 grains, just a third of
its bulk would be immersed; if 12,625 grains, half; if 16,832 grains, it
would sink two-thirds of its volume, and so on. Or, if, when the box is
floating, you make a mark upon its side at the exact level of the
surface of the water, the bulk of that portion of the box which lies
below the water-level can be ascertained. Suppose it to be thirty cubic
inches, then the weight of the box will be 30 × 252·5 or 7575 grains.
Hence it may be said that the immersed part of a floating body takes the
place of the water which it displaces, and, as it were, represents it.
If you press downwards upon the floating box, there is a feeling of
resistance as it descends, and when the pressure is taken off, the body
immediately rises again. Hence the water presses upwards against the
bottom of the floating body. But it also presses against the sides, for
if the sides of the box are very thin they will be driven in. If a thin
empty bottle is tightly corked and lowered into deep water the cork will
be driven in, or else the bottle will be crushed.