in all seas, it is only in the Southern Ocean that a sufficient
expanse of water exists for the tidal action to be fully
developed. This ocean has an average width of 1,500 miles, and
completely encircles the earth on a circumferential line 13,500
miles long; in it the attraction of the sun and moon raises the
water nearest to the centre of attraction into a crest which
forms high water at that place. At the same time, the water is
acted on by the centripetal effect of gravity, which, tending
to draw it as near as possible to the centre of the earth, acts
in opposition to the attraction of the sun and moon, so that at
the sides of the earth 90 degrees away, where the attraction of
the sun and moon is less, the centripetal force has more
effect, and the water is drawn so as to form the trough of the
wave, or low water, at those points. There is also the
centrifugal force contained in the revolving globe, which has
an equatorial diameter of about 8,000 miles and a circumference
of 25,132 miles. As it takes 23 hr. 56 min 4 sec, or, say,
twenty-four hours, to make a complete revolution, the surface
at the equator travels at a speed of approximately 25,132/24 =
1,047 miles per hour. This centrifugal force is always
constant, and tends to throw the water off from the surface of
the globe in opposition to the centripetal force, which tends
to retain the water in an even layer around the earth. It is
asserted, however, as an explanation of the phenomenon which
occurs, that the centripetal force acting at any point on the
surface of the earth varies inversely as the square of the
distance from that point to the moon, so that the centripetal
force acting on the water at the side of the earth furthest
removed from the moon is less effective than that on the side
nearest to the moon, to the extent due to the length of the