DECENNIAL PERIOD ON MAGNETIC VARIATIONS, ETC. 5938 
1862 may be connected with actions passing between the sun and another 
planet, such as Venus (the two most marked exceptional cases occupying about 
seven months). 
60. The discussions of sun-spot area relatively to planetary periods have all 
been undertaken on the hypothesis that the actions of the planets should be the 
same when they return to the same points of their orbits. No hypothesis has 
been proposed as to the distribution of the medium by which the solar electri- 
cal actions are supposed to be conveyed to the planets. In a paper read to the 
Royal Society of London, January 27, 1876, I have shown that marked mag- 
netic disturbances repeat themselves at intervals of exactly twenty-six days, in 
which cases there is a sudden diminution of the earth’s magnetic force. This 
interval is that obtained for the time of the sun’s rotation from numerous mag- 
netic observations. The result just mentioned proves that these disturbances 
cannot be related directly to the sun’s spots, whose time of rotation varies from 
twenty-six to nearly twenty-eight days. It also seems to show that the medium 
which conveys the solar action is not the same as that which transmits light and 
heat, since the repeated electrical actions are felt by the earth when the same 
solar meridian returns opposite the earth.* 
61. If we suppose that this electrical medium is derived from the sun, is 
of limited extent, unsymmetrically distributed around him, and has its own 
proper motion of rotation, it will be evident that no investigation for a given. 
planetary period could give the same result for the same part of the planet’s 
orbit. 
62. Whatever theory of the formation of sun-spots may be adopted, we shall 
always require it to explain why there are few or no spots in some years of 
the decennial period? Sir Joun HERSCHEL supposed that sun-spots may be 
produced in the same way as terrestrial cyclones. M. Faye has proposed an 
exceedingly rational hypothesis for their formation, depending on the diminish- 
ing velocities of contiguous zones on proceeding from the equator towards the 
poles. “Le décroissement,” says the distinguished French astronomer, “ bien 
plus rapide sur le soleil qwil ne le serait en vertu de la seule différence des 
rayons des paralleles de rotation,t donne naissance ¢a et la dans la photosphére 
a des tourbillons verticaux tout a fait analogues a ceux qui se produisent si 
* T may remark that M. Becqurren has proposed a hypothesis by which the atmospheric electri- 
city is derived from the sun, and has connected this derivation with the actions producing solar spots. 
—Comptes Rendus, Nov. 11, 1872, p. 1126. 
+ M. Favs explains the difference between the velocities shown by the spots in different lati- 
tudes and those which should result from difference of radii of the circles of latitude by a hypothesis, 
according to which the stratum where the falling incandescent rain is converted anew into vapour, has 
a different form than the external surface of the photosphere,—is, in fact, ellipsoidal, and flattened at 
the poles. It seems to me that were this double form consistent with dynamical laws, we should have 
here an indication of a different distribution of solar heat at the equator and at the poles, and a cause 
of currents between the two, which M. Fayz objects to in Sir J. Hurscuet’s hypothesis as non-existent, 
—Annuaire du Bureau des Longitudes, 1873, p. 516, 
