DECENNIAL PERIOD OF THE MAGNETIC VARIATIONS, ETC. 587 
responding increase of disturbance. From this time till 1861 there is a 
remarkable similarity in the variations, though they do not bear a constant 
ratio to each other. 
3d, In 1861 marked differences begin to appear ; and these are seen, espe- 
cially in.the great increase of sun-spot area in June 1862, when there is a mini- 
mum of disturbance. There is again, however, a considerable resemblance in 
the two curves during 1863, which disappears again in 1864. 
4th, On the whole, these curves show that the disturbances cannot be 
measured by, and are not due to sun spots; but the coincidences are too 
numerous to allow us to avoid the conclusion, that they are due to a common 
cause. This cause, however, may produce great changes of sun-spot area with- 
out any corresponding change in the amount of disturbance; and considerable 
changes of the amount of disturbance exist when there are no sun spots. 
45. Decennial Period of Magnetic Disturbance at different hours.—We have 
seen that the mean oscillation of the magnetic needle between the hour when 
the polar end of the needle attains its maximum westerly position and other 
hours follows approximately the same law as the total oscillation (Art. 35) ; 
it has seemed to me desirable to examine whether the irregularities of position 
at each hour also follow the decennial law. The following table contains these 
means for each hour in each year :— 
TABLE IV.—Houwrly Means of Disturbance of the Magnetic Needle at Trevandrum, 1854-1864. 


Hour. 1854. | 1855. | 1856. | 1857. | 1858. | 1859. | 1860. | 1861. | 1862. | 1863. | 1864. | 




124 a.m.|0-240 | 0-178 |0-140 | 0-200 | 0-237 | 0-305 | 0-204 | 0.267 | 0-253 | 0-202 | 0-237 
14 ,, | 0-261 |0-191 |0-151 | 0-219 | 0-260 | 0-311 | 0-304 | 0-283 | 0-255 | 0-237 | 0.287 
21°, | 0.272 | 0-209, | 0-150 | 0-219 | 0-282 | 0-335 | 0-317 | 0-311 | 0-268 | 0.265 | 0.322 
31, [0-279 | 0-225 | 0-159 | 0-232 | 0-302 | 0-358 | 0-349 | 0-310 | 0-280 | 0-290 | 0.341 
AL |, | 0.289 | 0-237 | 0-182 | 0-227 | 0-320 | 0-377 | 0-412 | 0-312 | 0-285 |0.315 | 0.339 
5k ,, |0-316 | 0.272 | 0-209 | 0-247 | 0-374 | 0-437 | 0-441 | 0-354 | 0.324 | 0.318 | 0.362 
GL ,, |0-376 | 0-333 | 0-332 | 0-372 | 0-443 | 0-565 | 0-579 | 0.493 | 0-438 | 0-440 | 0.436 
71 ,, [0-432 | 0-401 | 0-388 | 0-446 | 0-495 | 0-612 | 0-619 | 0-568 | 0-485 | 0.497 | 0.495 
8h ,, | 0-465 | 0-468 | 0-438 | 0-523 | 0-567 | 0.677 | 0-678 | 0.606 | 0-519 | 0-558 | 0.518 
91 ,, [0-507 | 0-535 | 0-506 | 0-597 | 0-623 | 0-757 | 0-758 | 0-677 | 0-577 | 0-589 | 0-556 
104 ,, | 0-562 | 0-562 | 0-566 | 0-613 | 0-672 | 0-765 | 0-824 | 0-702 | 0-648 | 0.622 | 0.589 
114 ,, | 0-594 | 0-560 | 0-570 | 0-596 | 0-677 | 0-826 | 0-790 | 0-722 | 0-695 | 0.636 | 0-579 
01 p.m.| 0-590 |0-548 | 0-551 | 0-608 | 0-699 | 0-815 | 0-784 | 0-758 | 0-693 | 0-650 | 0-602 
14 ,, |0-564 |0-521 | 0-517 | 0-575 | 0-723 | 0-764 | 0-760 | 0-698 | 0-693 | 0-612 | 0-627 
21 ,, |0-536 | 0-483 | 0-453 | 0-526 | 0-662 | 0-707 | 0-698 | 0-626 | 0-639 | 0-623 | 0-592 
31 ,, |0-503 | 0-431 | 0-410 | 0-487 | 0-611 | 0-647 | 0-642 | 0-558 | 0-593 | 0-590 | 0.562 
42 ,, | 0-422 | 0-370 | 0-359 | 0-414 | 0-498 | 0-572 | 0-567 | 0-500 | 0-534 | 0-544 | 0-497 
5h ,, |0-317 | 0-293 | 0-289 | 0-337 | 0-272 | 0-521 | 0-469 | 0-417 | 0-428 | 0.449 | 0-432 
61 ,, | 0-250 | 0-234 | 0-202 | 0-264 | 0-317 | 0-449 | 0-371 | 0-336 | 0-366 | 0-365 | 0-357 
72 ,, |0-224 | 0-217 | 0-182 | 0-242 | 0-304 | 0-394 | 0-359 | 0-311 | 0-334 | 0-341 | 0-292 
81 ,, | 0-212 | 0-208 | 0-163 | 0-201 | 0.274 | 0-394 | 0-307 |.0-295 | 0-308 | 0-305 | 0.256 
91 ,, | 0-200 | 0-190 | 0-148 | 0-196 | 0-258 | 0-357 | 0-298 | 0-245 | 0-293 | 0-270 | 0-219 
10: ,, [0-199 | 0-172 | 0-137 |0-193 | 0-242 | 0-347 | 0-267 | 0-247 | 0-249 | 0-237 | 0-214 
113 ,, |0-215 |0-168 | 0-137 | 0-187 | 0-237 | 0-310 | 0-257 | 0-257 | 0-266 | 0-219 | 0-202 











Note.—The means at the half hours for 1854 and 1855 are interpolated from the observations 
at the whole hours. 
VOL. XXVII. PART Iv. 70 
