CarruTHERs.—Retardation of Earth's Rotation by Volcanic Action. 858 
when subjected to strain, more or less, and there must, therefore, be an 
actual daily tide in the land as well as in the ocean. If the earth had no 
more rigidity than water, there would be no rise and fall of the sea on the 
shore, for the land would rise as fast as the water. Our tides are the dif- 
ference between the land and water tides; they prove and measure the 
rigidity of the crust. We know from them that the earth is rigid, and that 
it does not yield to strains, tending to change its form in the way a molten 
or viscid mass would do. 
We will now examine the effect of this land tide in order to ascertain 
the kind and relative amounts of the strains to which it exposes the crust. 
The tide due to the sun alone is about two feet; that due to the moon 
alone is about five feet, or seven feet altogether. The major axis of the equa- 
torial tidal ellipse is, therefore, at spring tides, fourteen feet longer than the 
minor axis, or at least it tends to become so. The distance measured on 
the surface through the pole is twenty-one (21) feet longer from one end to 
the other of the major axis of the tidal ellipse, than from one end to the 
other of the minor axis. 
By the earth’s rotation the ssi of the major axis passes away from 
under the sun and moon, and the end of the minor axis takes its place. 
The major then becomes the minor axis, and must tend to shorten fourteen 
feet, while the minor tends to lengthen the same amount. Part of the 
crust tends to stretch and part to compress twenty-one feet. 
The polar axis of the earth undergoes no alteration in length from these 
changes, but if the Sun and Moon in their motion in the heavens separate 
and come into quadratures, the tide instead of being seven fect is reduced 
to three feet. No change would take place in the strains above described, 
but the polar semi axis would be lengthened two feet, thus introducing new 
and very important tensile strains. 
We thus see that the crust of the earth is subjected to a racking move- 
ment which exposes every part of it to a tensile and compressive strain 
alternately, and that the polar axis is continually lengthening or shortening 
or at least tending to do so. : 
The crust does not break under these strains, but if another action 
were added, which increased indefinitely the tensile strains caused by the 
tides, it will readily be perceived that fracture must take place sooner or 
later. Such an action does exist in the retardation of the earth’s rotation, 
by which the strains due to the elongation of the polar axis are indefinitely 
increased. 
The earth owes its spheroidal form to its rotary motion; the compres- 
sion of the poles depends on the velocity of rotation sak varies as its 
square. If, therefore, anything reduces this velocity, the polar axis must 
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