OK THE NUMERICAL RELATIONS OF GRAVITY AND MAGNETISM. 



123 



It therefore appears that 



1. The night fluctuations of magnetic force (except during the four hours after sun- 

 set) arc, as their supposed relations to gravity would lead us to expect, comparatively 

 insignificant. 



2. The magnetic influence of the atmospheric rotation-tide is perhaps shown by the 

 convergence of the lines of equal magnetic disturbance near the hours of normal high 

 barometer, and their divergence near the hours of low barometer. Thus, in the night, 

 the difference of force is as great in the two hours between 9 and 11 P.M., as in the three 

 hours between 2 and 5 A. M. ; and in the day the difference in the three hours between 8 

 and 11 a.m. is two greater than in the four hours between 1 and 5 p.m. 



3. The abnormal hourly differences (C — I)) appear to be intimately connected with the 

 barometric tides, having, like the latter, and at corresponding hours, two quarter-daily 

 phases of excess and two of deficiency. 



4. Not only do the times of mean baric and magnetic disturbance correspond very closely, 

 as we have seen, but the observed hourly magnetic variation at the times of solar low- 

 tide (6-7 a.m. and 6'-7 p.m., allowing for the disturbance of rotation) is nearly the same 

 as the theoretical variation (C — D =, — 1 in each instance). 



5. In the hour immediately following the times of solar mean-tide (3 h ' s 9 h -, 15 1 '-, and 

 2P-), the average theoretical and observed hourly differences are the same. 



0, . 

 I),. 



C— 1), 



3 P. M. 



9 P. M. 



;i a. m. 



9 A. M. 



Average 



—20 



—4 



+ 1 



+29 



1.5 



—15 



—4 



+ 7 



+ 18 



1.5 



+11 



6. If any further evidence were necessary to convince us that the barometric and mag- 

 netic disturbances are more directly dependent upon rotation, and the consequent con- 

 tinually changing position of each point of the earth's surface relatively to the sun, than 

 upon mere changes of temperature,* it might be drawn from a comparison of columns 7, 

 8, and 9, in Table IV, or obtained by taking the half-daily barometric differences at each 

 hour, as below : 



P.M., 

 A.M. 



I) h. 



164 

 92 



72 



I h. 



—2 



—44 

 42 



i h. 

 —161 



—175 



14 



;s b. 

 —268 

 —259 



—9 



4h. 

 —300 



—271 



—29 



5 h. 

 —259 



—210 

 —49 



(i h. 



—179 



—84 



7h. 



-57 

 77 



8 h. 



78 

 227 



'.I h. 



182 



—95 



-134 



342 

 — 149 —100 



10 h. rih. 

 240 204 



363 



296 

 -123 —92 



* I speak hero merely of I he disturbances. The total force of terrestrial magnetism, according to my theory, 

 is principally dependent upon the gravitation of temperature currents; but I think that the daily perturbations, 

 both of the magnet ami barometer, are determined almost entirely by ohanges of position oonsequent upon rotation. 



