DIURNAL VARIATION OF THE BAROMETER. 
BY 
Alexander Smyth Christie. 
[Read before the Society March 12,1892.] 
The moon and sun, attracting according to Newton’s law, 
raise tides in the earth’s atmosphere as well as in its tidal 
waters, and the periods of the former as well as of the latter 
tides depend upon the periods of terms in the Newtonian 
potential of the disturbing bodies. That these atmospheric 
tides have not been put more clearly in evidence must be 
due to the employment of defective and insufficient methods 
of search; or to their being in part transmuted, as fast as 
they are formed, into other species of motion, to subserve 
other ends in the economy of nature; or to both of these 
causes combined. I have no doubt that barometric records 
are obtained sufficiently precise to enable us to find these 
tides, using an appropriate analysis; but the only adequate 
analysis is the tidal harmonic. To indicate how grossly 
inadequate is a distribution according to the lunar day or 
half day, or to the solar day or half day, I may say that no 
one of these distributions provides for finding such impor¬ 
tant components as the lunar diurnal, solar diurnal, and 
luni-solar diurnal, which constitute almost wholly any diur¬ 
nal tide arising from the attraction of moon and sun, a tide 
which must be relatively large at many barometric stations. 
It may be further remarked that the part of the variation 
of the barometric column not directly due to the gravita¬ 
tional attraction of moon and sun depends, in one way or 
another, upon the rotation of the earth and the motions of 
11—Bull. Phil. Soc., Wash., Vol. 12. 
( 67 ) 
