68 
CHRISTIE. 
sun and moon. Suppose that this part of the variation is 
due to oscillations of the atmosphere set up by the thermal 
power of the sun: it must consist of at least two waves, whose 
fundamental periods are a solar day and a tropical year, 
respectively, corresponding to like thermal waves; and the 
relative and absolute magnitudes of these fluctuations render 
probable the formation of combination waves,* of which 
the two principal ones would coalesce with and reinforce 
th,e solar diurnal and luni-solar diurnal attractional; and in 
whatever other way the barometric variations depend upon 
the sun and moon, they will be resolvable into simple cosine 
waves which depend upon the earth’s rotation and the mo¬ 
tions of sun and moon; these cosine waves, with periods 
assigned a priori , being the natural and appropriate elements 
of the variations and readily picked up, if of sensible magni¬ 
tude, by the methods of the tidal harmonic analysis. 
It is also to be said that in so far as the terrestrial mag¬ 
netic elements depend upon the tidal motions of the atmos¬ 
phere,t or upon the action of the sun and moon as magnets, 
or upon the sun’s thermal power, the appropriate analysis 
of observations of these elements is the tidal harmonic; and 
it is worth while to observe that if any part of the magnetic 
phenomena arises from the tidal action of sun and moon, as 
suggested by Professor Schuster in the paper cited in the 
note, the barometric variations dependent upon that tidal 
action will be correspondingly diminished and less easily 
detected. 
* Helmholtz, Berlin Monatsberichte, May, 1856, and Poggendorff’s Anna- 
len, vol. 99, pp. 497-540; or Wiss. Abhand., vol. 1, pp. 256-302; or his Lehre 
v. den Tonempfindnngen (Ch. VII). 
11 have considered this suggested dependence of the magnetic varia¬ 
tions upon tidal motions only since reading the above, Professor H. A. 
Hazen having kindly referred me to Professor Schuster’s important paper 
On the Diurnal Variation of Terrestrial Magnetism (Phil. Trans., vol. 180), 
which contains, inter alia , the conclusion that “ the horizontal movements 
in the atmosphere which must accompany a tidal action of the sun or 
moon or any periodic variation of the barometer such as is actually ob¬ 
served would produce electric currents in the atmosphere having magnetic 
effects similar in character to the observed daily variation.” 
