
DECENNIAL PERIOD OF MAGNETIC VARIATIONS, ETC. 585 
rectly on the sun,* and the amplitudes of both the solar and lunar diurnal varia- 
tions have been found to obey the decennial law.t The means of all the differ- 
ences may therefore be considered due to direct or indirect solar disturbance. 
The yearly means are given in the following table :— 
TaBeE III.t— Yearly Means of Solar Disturbance of Magnetic Declination corresponding to 
the 1st of each Month.—Trevandrum, 1854-1864. 

Year. Jan. Feb. Mar. | April. | May. | June. | July. | Aug. | Sept. Oct. Noy. Dee. 





Monnaie ee he & Pei ... |0-368 | 0-358 | 0-347 | 0-345 | 0-338 | 0-334 
1855 | 0-336 | 0-331 | 0-336 | 0-338 | 0-335 | 0-335 | 0-334 | 0.339 | 0-339 | 0-331 | 0-327 | 0-323 
1856 | 0-316 | 0-315 | 0-314 | 0-310 | 0-305 |0-305 | 0-306 | 0-300 | 0-298 | 0-299 | 0.304 |0-317 
1857 | 0-321 | 0-325 | 0-322 | 0-331 | 0-337 | 0.344 | 0-363 | 0-380 | 0-390 |0-404 | 0-419 | 0-415 
1858 | 0-420 | 0.424 | 0-432 | 0-431 | 0-437 | 0-442 | 0-436 | 0-434 | 0-448 | 0-446 |0-445 | 0.447 
1859 | 0-451 | 0-460 | 0-477 | 0-490 | 0-514 10-516 | 0-525 |0-523 |0-519 | 0-533 | 0-534 | 0.540 
1860 | 0-541 | 0-551 | 0-553 |0-547 | 0-524 |0-516 | 0-505 |0-503 | 0-495 | 0-484 | 0-478 | 0-468 
1861 | 0-466 | 0-450 | 0-437 | 0-436 | 0-441 | 0.447 | 0-453 | 0-450 | 0-450 | 0-447 | 0-443 | 0-439 
1862 | 0-433 | 0-433 | 0-433 | 0-435 |0-439 | 0-436 | 0.434 |0-447 | 0-449 | 0-453 |0-457 | 0-463 
1863 | 0-466 | 0-462 | 0-459 | 0-451 | 0-440 | 0-435 | 0-424 |0-406 | 0-406 | 0-398 | 0-398 | 0-399 
1864 [0-406 | 0-408 | 0-407 | 0-408 | 0-408 | 0-411 | 0-414 |... pe, eeaiouTe. Soleo 















The means of Table III. are projected (Plate X X XIX.) immediately below the 
yearly mean sun-spot area. I have also projected below the yearly mean ~ 
diurnal range, the yearly mean areas of the curves representing the monthly 
mean diurnal variations; the latter curve agrees to a great extent with that 
of the ranges. 
41. The disturbance curve gives the following epochs,—minimum, 1856°7 ; 
maximum, 1860°2. These differ little from the epochs derived from the ranges, 
the minimum by the latter occurring, however, nearly five months later. The 
two curves do not resemble each other exactly, and there is a tendency to the 
maximum, shown by the sun-spot area at 1859-2, which is not shown by the 
ranges. 
42. When we seek the ratio of the maximum to the minimum yearly mean 
disturbance we find 
1860°2 0°553 
18507’ 0-293 — 168. 

The ratio for the disturbance is therefore greater than for the amplitudes of 
the diurnal variations.§ 
* Trans. Roy. Soc. Edin., vol xxvi. p. 735. See also Trevandrum Magnetical Observations, Art. 
398, p. 133, and Appendix, Art. 200, p. 544. 
t Trev. Mag. Obs., vol. i. p. 120. 
+ Derived from Table LI., “‘ Trevandrum Magnetic Observations,” vol. i. p. 142. The two follow- 
ing means require correction in the table cited. Yearly means for 1864, for 0:406 read 0°414, and 
mean for July, for 0°399 read 0°406. 
§ It is well known that magnetic disturbances are most felt in high latitudes; it would be of im- 
