254 W. B. Taylor — Crumpling of the Earth 's Crust. 



dilations of this chapter, the retarding pressure of the tides 

 against the earth's rotation, would cause — during the lapse of 

 2,500 years — a sidereal day to be lengthened to the extent of 

 one-sixteenth of a second. As the length of the day however 

 has remained constant, the cooling effect of the earth during 

 the same period of time, must have shortened the day one- 

 sixteenth of a second. A diminution of the earth's radius to 

 the amount of 4£ meters in 2,500 years, corresponds to this 

 effect. Hence in the course of the last 25 centuries, the tem- 

 perature of the whole mass of the earth must have decreased 

 one-fourteenth of a degree [C.]."* In this ingenious adjustment, 

 the eminent philosopher doubtless greatly over-estimated the 

 earth's cooling and consequent contraction, in order to com- 

 promise the "difficulties of the physicist" with the authority 

 of the astronomer. 



In 1853, our colleague Mr. William Ferrel in a paper " On 

 the effect of the Sun and Moon upon the rotatory motion of the 

 Earth," independently- undertook the mathematical investiga- 

 tion : the very name of Mayer being then (it is scarcely neces- 

 sary to state) as wholly unknown in this country as it was in 

 England. In this important paper it is shown that external 

 forces of the second order — neglected by Laplace as producing 

 no sensible effect upon the rotation period, are — though very 

 small — decidedly appreciable, and " must, if not counteracted 

 by some other effect, produce a sensible variation in the earth's 

 rotatory motion." Assuming a lunar ocean tide of two feet, with 

 a lag of two hours, Mr. Ferrel estimated an equatorial retarda- 

 tion of 37 miles per century, increased by the solar tide to 44 

 miles per century, which would give an apparent lunar acceler- 

 ation of V 24" in a century. "As no such acceleration has 

 been observed — above what is accounted for otherwise, the 

 rotatory motion of the earth must be nearly uniform ; and the 

 above effect of the sun and moon must be accounted for by the 

 gradual contraction of the earth through a loss of temperature." 

 This would require a subsidence of the surface, or a reduction 

 of the earth's radius (supposing the contraction equable through 

 the mass), of about one foot per century.f 



In the same year, a few months earlier, Mr. John C. Adams 

 in a careful memoir " On the secular variation of the moon's 



*L. E. D. Phil. Mag., May and June, 1863, vol. xxv, pp.403, and 423-427. 

 Also Am. Jour. Sci., 1864, vol. xxxviii, pp. 398, and 409-413. 



f Gould's Astronom. Jour., Dec. 8, 1883, vol. iii, pp. 138-140. In this paper 

 (p. 141), Mr. Ferrel shows that the similar retarding effect of the earth upon a 

 primitive rotation of the moon (supposed originally fluid) must be 562 times 

 greater than the converse earth retardation ; and that this therefore sufficiently 

 explains the present reduction of the moon's rotation to the period of a lunar 

 month. This is believed to be the first suggestion of the now accepted cause of 

 this peculiarity of the moon. Mr. Ferrel's paper is dated September, 1853. A 

 similar explanation was offered by Helmholtz in the following year. 



