B. TERRESTRIAL PHYSICS AND METEOROLOGY. 21 



ences, thus leaving a very much smaller difference to be 

 shown- by water if placed on the surface of such a globe, and 

 estimated in its rise and fall relatively to the solid bottom on 

 which it rested. The precise agreement of precession and 

 nutation, with dynamical estimates founded on the supposi- 

 tion of the earth being perfectly rigid, made it probable that 

 the earth was, in reality, vastly more rigid, as a w T hole, than 

 any specimen of surface rock in the condition in which it is 

 when experimented on in our laboratories. The proposed 

 tidal observation and calculation he considers to be the only 

 method which gives directly, and without any possibly doubt- 

 ful suppositions regarding interior arrangement of density on 

 the earth, a measurement of its elastic yielding to the tide- 

 generating influences. 15 A,Angnst 19, 1871, 237. 



CEOLL OX THE ACTION OF TIDES OX THE EAETH. 



Mr. James Croll, well known for his valuable papers upon 

 ocean currents and other physical phenomena, remarks, in 

 JSfature, upon the discussions which have lately taken place 

 in regard to Sir William Thomson's conclusion, that had the 

 earth solidified several millions of years ago, when it must 

 have been rotating much more rapidly than at present, its 

 form should have been different from what it actually pre- 

 sents; or, in other words, there should have been a much 

 greater difference than now exists between the equatorial 

 and polar diameters. Regarding all the other arguments ad- 

 vanced by Sir William Thomson in regard to the age of the 

 globe as unassailable, Mr. Croll does not agree to the conclu- 

 sion from tidal retardation, but considers the real objection 

 to the argument to be as follows : as the rate of rotation de- 

 creases under tidal retardation, centrifugal force must de- 

 crease also. The consequence, therefore, is that the sea must 

 be slowly sinking at the equator and rising at the poles. But 

 denudation is also lowering the land at the equator, and 

 therefore the whole question concentrates itself in this : Will 

 the denudation lower the level of the land at the equator as 

 rapidly as the sea sinks ? This question, hapjrily, can be an- 

 swered. The method lately discovered of measuring the rate 

 of subaerial denudation enables us to determine the rate at 

 which the land at the equator is lowered ; and from the prin- 

 ciples of mechanics, the rate at which the sea is sinking at the 



