i.e._ in one hour
twenty-five minutes, instead of twenty-four hours), bodies at the
equator would have such a strong tendency to fly outwards that the force
of terrestrial gravity acting upon them would just be counterpoised, and
they would virtually have _no weight_. And, further, were the earth to
rotate a little faster still, objects lying loose upon its surface would
be shot off into space.

The earth is, therefore, what is technically known as an _oblate
spheroid_; that is, a body of spherical shape flattened at the poles. It
follows of course from this, that objects at the polar regions are
slightly nearer to the earth's centre than objects at the equatorial
regions. We have already seen that gravitation acts from the central
parts of a body, and that its force is greater the nearer are those
central parts. The result of this upon our earth will plainly be that
objects in the polar regions will be pulled with a slightly stronger
pull, and will therefore _weigh_ a trifle more than objects in the
equatorial regions. This is, indeed, found by actual experiment to be
the case. As an example of the difference in question, Professor Young,
in his _Manual of Astronomy_, points out that a man who weighs 190
pounds at the equator would weigh 191 at the pole. In such an experiment
the weighing would, however, have to be made with a _spring balance_,
and _not with scales_; for, in the latter case, the "weights" used would
alter in their weight in exactly the same degree as the objects to be
weighed.

It used to be thought that the earth was composed of a relatively thin
crust, with a molten interior. Scientific men now believe, on the other
hand, that such a condition cannot after all prevail, and that the earth
must be more or less solid all through, except perhaps in certain
isolated places where collections of molten matter may exist.

The _atmosphere_, or air which we breathe, is in the form of a layer of
limited depth which closely envelops the earth. Actually, it is a
mixture of several gases, the most important being nitrogen and oxygen,
which between them practically make up the air, for the proportion of
the other gases, the chief of which is carbonic acid gas, is exceedingly
small.

It is hard to picture our earth, as we know it, without this atmosphere.
Deprived of it, men at once would die; but even if they could be made to
go on living without it by any miraculous means, they would be like unto
deaf beings,