Chase.] 



582 



[Nov. 4, 



account for such difference by any theory of magnetic induction.* It is 

 evident, however, that Sun's thermal disturbance of Earth's gethereal cur- 

 rents is greater than Moon's, while Moon's tidal disturbance of the same 

 currents is greater than Sun's. 



117. Magnetic Variations at St. Helena. 



St. Helena, on account of its insular position, its proximity to the mag- 

 netic equator, its large horizontal force and the large proportion of total 

 force which is represented by the horizontal force, is free from many of 

 the local complications which often mask the normal action of the Sun and 

 Moon. Moreover, the comparatively long period of systematic observa- 

 tions, the extension of the observations to the lunar action on the baro- 

 meter as well as on the magnetic needle, the uniformity of the indications 

 in different years and in difterent semesters, and the symmetry which 

 is traceable between the lunar atmospheric and magnetic disturbances, are 

 additional reasons for regarding it as a typical station for the study of 

 gravitating influence on aethereal waves. 



118. Comparative Table. 



The difference between the solar and lunar disturbances is shown in the 

 following synoptical table. The magnetic data are taken from the second 

 volume of the St. Helena Observations, pp. xlii-xliv and Iviii-lxii ; the 

 barometric, from the first volume, pp. 84, 99. The barometric ratios are 

 given, in order to make the table homogeneous and facilitate comparison. 

 They were obtained by dividing the hourly deviations from the mean 

 by the mean height of the barometer (28.278 in.). 

 Solar Disturbances. 



*See paper by Dr. Lloyd, Proc. R. Irish Acad., 1858, and C. Chambers, Phil. 

 Trans., 1863. 



