ey would go travelling across the earth, tidal waves
three feet high, and constituting the mid-ocean tides. But in the
northern hemisphere they can only thus journey a little way without
striking land. As the moon rises at a place on the east shores of the
Atlantic, for instance, the waters begin to flow in towards this place,
or the tide begins to rise. This goes on till the moon is overhead and
for some time afterwards, when the tide is at its highest. The hump then
follows the moon in its apparent journey across to America, and there
precipitates itself upon the coast, rushing up all the channels, and
constituting the land tide. At the same time, the water is dragged away
from the east shores, and so _our_ tide is at its lowest. The same thing
repeats itself in a little more than twelve hours again, when the other
hump passes over the Atlantic, as the moon journeys beneath the earth,
and so on every day.

In the free Southern Ocean, where land obstruction is comparatively
absent, the water gets up a considerable swing by reason of its
accumulated momentum, and this modifies and increases the open
ocean tides there. Also for some reason, I suppose because of the
natural time of swing of the water, one of the humps is there
usually much larger than the other; and so places in the Indian and
other offshoots of the Southern Ocean get their really high tide
only once every twenty-four hours. These southern tides are in fact
much more complicated than those the British Isles receive. Ours
are singularly simple. No doubt some trace of the influence of the
Southern Ocean is felt in the North Atlantic, but any ocean
extending over 90 deg. of longitude is big enough to have its own
tides generated; and I imagine that the main tides we feel are thus
produced on the spot, and that they are simple because the
damping-out being vigorous, and accumulated effects small, we feel
the tide-producing forces more directly. But for authoritative
statements on tides, other books must be read. I have thought, and
still think, it best in an elementary exposition to begin by a
consideration of the tide-generating forces as if they acted on a
non-rotating earth. It is the tide generating forces, and not the
tides themselves, that are really represented in Figs. 112 and 114.
The rotation of the earth then comes in as a disturbing cause.