Chase.] 



494 



[December. 



VII) is slightly greater than the corresponding observed variation. 

 The excess in one case exactly counterbalances the deficiency in the 

 other, the sum of the theoretical being precisely equal to the sum of 

 the observed variations. 



IX. The total daily magnetic variations, like the barometric, can 

 be resolved into a variety of special tides, which may be severally 

 explained by well-known constant or variable current-producing and 

 weight-disturbing forces. 



B 



Theoretical 

 DiflFerential 

 Solar Tide. 



+ .00024 

 —.00024 

 + .00024 



The hours are counted from midnight, in each half-day. 



Column A contains the hourly differences from mean weight, attri- 

 butable to solar gravitation, with changed signs ; diminution of weight 

 being accompanied with increase of magnetism, and vice versa. 



The form of the tide in column B is evidently such as should be 

 determined by solar action. The magnitude of the tide is estimated 

 by comparing the relative amounts of motion down the diagonal and 

 down the arc of a quadrant (.00067x[l— ( f — i)]=.00048). 

 The mean-tidal difference [(.00067— .00048)-i-2] is very nearly 

 equivalent to the average theoretical inertia-disturbance of weight. 

 The atmospheric inertia at St. Helena, (regarding the fluctuations 

 as uniform between successive hourly observations), produces re- 

 tardations of 59', 85', 26', and 31', at Oh., 6h., 12h., and 18h., re- 

 spectively. The mean retardation is 50', or ^^j of a half-day. The 

 theoretical daily gravity-variation being .00134, the average varia- 

 tion in ^^ of a half-day is .00009^^, the mean tidal difference being 

 .00009J. 



The consideration of the moon's disturbance of the atmospheric 

 gravitation, is complicated by the magnitude of its differential attrac- 

 tion, the position of the centre of gravity of the terrestrial system, 

 the varying centrifugal force, and other circumstances involved in 

 the lunar theory. Still there are indications in the following synop- 

 sis, of the influence of gravity, sufl&ciently striking to encourage a 

 hope that our knowledge of the moon's perturbations may be im- 

 proved by a thorough comparative study of the lunar atronomical, 

 atmospheric, and magnetic tables. 



