verything at the equator is thus moving along at the rapid rate of
about 1000 miles an hour, or between sixteen and seventeen times as
fast as an express train. If, however, one were to take measurements
around the earth parallel to the equator, one would find these
measurements becoming less and less, according as the poles were
approached. It is plain, therefore, that the speed with which any point
moves, in consequence of the earth's rotation, will be greatest at the
equator, and less and less in the direction of the poles; while at the
poles themselves there will be practically no movement, and objects
there situated will merely turn round.

The considerations above set forth, with regard to the different speeds
at which different portions of a rotating globe will necessarily be
moving, is the foundation of an interesting experiment, which gives us
further evidence of the rotation of our earth. The measurement around
the earth at any distance below the surface, say, for instance, at the
depth of a mile, will clearly be less than a similar measurement at the
surface itself. The speed of a point at the bottom of a mine, which
results from the actual rotation of the earth, must therefore be less
than the speed of a point at the surface overhead. This can be
definitely proved by dropping a heavy object down a mine shaft. The
object, which starts with the greater speed of the surface, will, when
it reaches the bottom of the mine, be found, as might be indeed
expected, to be a little ahead (_i.e._ to the east) of the point which
originally lay exactly underneath it. The distance by which the object
gains upon this point is, however, very small. In our latitudes it
amounts to about an inch in a fall of 500 feet.

The great speed at which, as we have seen, the equatorial regions of
the earth are moving, should result in giving to the matter there
situated a certain tendency to fly outwards. Sir Isaac Newton was the
first to appreciate this point, and he concluded from it that the earth
must be _bulged_ a little all round the equator. This is, indeed, found
to be the case, the diameter at the equator being nearly twenty-seven
miles greater than it is from pole to pole. The reader will, no doubt,
be here reminded of the familiar comparison in geographies between the
shape of the earth and that of an orange.

In this connection it is interesting to consider that, were the earth to
rotate seventeen times as fast as it does (_