reason why high tide is usually about twenty-five minutes later each
time.

The moon is, however, not the sole cause of the tides, but the sun, as
we have said, has a part in the matter also. When it is new moon the
gravitational attractions of both sun and moon are clearly acting
together from precisely the same direction, and, therefore, the tide
will be pulled up higher than at other times. At full moon, too, the
same thing happens; for, although the bodies are now acting from
opposite directions, they do not neutralise each other's pulls as one
might imagine, since the sun, in the same manner as the moon, produces a
tide both under it and also at the opposite side of the earth. Thus both
these tides are actually increased in height. The exceptionally high
tides which we experience at new and full moons are known as _Spring
Tides_, in contradistinction to the minimum high tides, which are known
as _Neap Tides_.

The ancients appear to have had some idea of the cause of the tides. It
is said that as early as 1000 B.C. the Chinese noticed that the moon
exerted an influence upon the waters of the sea. The Greeks and Romans,
too, had noticed the same thing; and Caesar tells us that when he was
embarking his troops for Britain the tide was high _because_ the moon
was full. Pliny went even further than this, in recognising a similar
connection between the waters and the sun.

From casual observation one is inclined to suppose that the high tide
always rises many feet. But that this is not the case is evidenced by
the fact that the tides in the midst of the great oceans are only from
three to four feet high. However, in the seas and straits around our
Isles, for instance, the tides rise very many feet indeed, but this is
merely owing to the extra heaping up which the large volumes of water
undergo in forcing their passage through narrow channels.

As the earth, in rotating, is continually passing through these
tide-areas, one might expect that the friction thus set up would tend to
slow down the rotation itself. Such a slowing down, or "tidal drag," as
it is called, is indeed continually going on; but the effects produced
are so exceedingly minute that it will take many millions of years to
make the rotation appreciably slower, and so to lengthen the day.

Recently it has been proved that the axis of the earth is subject to a
very small displacement, or rather, "wobbling," in the course of a
period of somewhat over a