Cliase.] 5ob fNov. 4, 



— .000067 = .000139) is almost identical with the range of lunar baromet- 

 ric disturbances (from — .000075 to + .000062 = .000137). 



The culminations of the lunar disturbances, both of the vertical and of 

 the horizontal force, correspond approximately with the mean sum of the 

 accelerations and retardations of lunar tidal action by terrestrial rotation 

 (6h -- v/S =r 4h 14.6m after Oh, 6h, 12h, 18h.) 



Vertical Force. Horizontal Force. 



1st Max. 0h + 3h57m , 1st Min. Oh. + 4h 1.3m 



1st Min. 6h + 4h 51.5m ' 1st Max. 6h. + 4h 24.4m 



2d Max. 12h + 3h 36.1m 2d Min. 12h. + 3h 44m 



2d Min. 18h + 4h 39.8m 2d Max. 18h. + 4h 9.2m 



Mean 4h 16.1m Mean 4h 4.7m 



122. Conclusions. 



Although the barometric observations furnish the most ready data for 

 quantitative measurements and comparisons, the combined action of terres- 

 trial rotation with lunar tidal and terrestrial equilibrating gravitation is 

 not confined to the air. Every particle of the globe is continually subject 

 to cyclical variations of stress and strain. In the first and third quadrants 

 the lunar action is opposed, while in the second and fourth it is aided, by 

 terrestrial rotation, so that the resultant of all the subterranean magnetic 

 influences must be subject to lunar disturbances of the same character as 

 those which modify the barometric and electric currents in the atmosphere. 



We may, therefore, conclude that the solar disturbance of the terrestrial 

 magnetic currents is chiefly and primarily due to its thermal activity ; the 

 lunar, to gravitating currents which are modified by terrestrial rotation 

 and orbital revolution. 



123. "Forced Oscillation.'" 



In discussing the synchronism of the motion of the moon's nodes with 

 terrestrial nutation Herschel* introduces "the principle of forced oscilla- 

 tions, or of forced vibrations," by the following announcement : 



"If one part of any system connected either by material ties, or by the 

 mutual attractions of its members, be continually maintained by any cause, 

 whether inherent in the constitution of the system or external to it, in a state 

 of regular periodic motion, that motion will be propagated throughout 

 the whole system, and will give rise, in every member of it and in everj^ 

 j)art of each member, to periodic movements executed in equal period 

 with that to which they owe their origin, though not necessarily synchro- 

 nous with them in their maxima and minima." 



A demonstration of this theorem for the forced vibrations of systems con- 

 nected by material ties of imperfect elasticity, is given in Herschel' s Treatise 

 on Sound.f Fourier's theorem, Herschel' s theory of the consequences of 



* Op. cit., Sect. 650. 



t Encyc. Metrop., Art. 323. 



