Prof. Challis on a Theory of Magnetic Force. 103 
tion of these motions, it must be concluded, in conformity with 
what is said in art. 28, that there is no perceptible effect from 
motion of translation of the atoms relative to the ether. 
34, The terrestrial streams will be disturbed by the streams 
due to the gyratory motion about the sun, and the direction and 
amount of the resultant, at a given place, of the combination of 
the solar and terrestrial streams, will vary with the time of day. 
Hence there will be a diurnal variation of magnetic intensity, as 
is also known from observation. The epoch of maximum in- 
tensity will depend in part on the position of the place relative 
to the magnetic poles. 
35. The amount of the variation of intensity on a given 
day will depend on the sun’s declination, and therefore will be 
subject to an annual variation, being in north latitudes greatest 
at the summer solstice, and least at the winter solstice. These 
results are confirmed by observation. 
It may deserve to be considered whether the magnetic in- 
tensity is sensibly affected by currents which must be produced 
in the atmosphere by solar heat, setting in all directions from 
the parts of the atmosphere most heated by the sun. The effeet 
of these currents would be subject to diurnal and annual varia- 
tions, and if of considerable amount, would have an influence on 
the local hour of maximum intensity. 
36. There will also be an annual variation of intensity inde- 
pendent of the sun’s declination, being due solely to the change 
of the velocity of the solar gyrations, caused by change of the 
-earth’s distance from the sun. Such a variation has been 
detected ; but its amount is very small. 
37. Through the telescope the sun presents itself to our view 
asa globular body, surrounded by a thick envelope consisting 
of matter which accumulates and divides, floatmmg and whirling 
most probably by the action of an atmosphere, and which is 
separated from the interior solid globe by an intervening space. 
‘We cannot be far wrong in calling such matter cloud. It is 
then reasonable to suppose that the observed changes are attri- 
butable to the action of galvanic or magnetic streams, generating 
and precipitating from time to time the cloudy matter. But 
changes can hardly be attributed to the proper magnetic streams 
of the sun, because these, as in the case of the earth, will most 
probably be of a constant character. The changes are rather 
due to disturbances of the solar streams by external influence, as 
‘by the gyratory streams of the planets. It is therefore quite 
in accordance with this theory to find, as has been done lately, 
that observations indicate periods of maxima and minima of 
solar spots. 'These periods will be determined by changes of the 
combined action of the gyratory motions about the planets, as 
