64 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVI.) 
fining- the attention to a spot where the sun is vertical, for the purpose of considering 
the condition of the atmosphere there and at other parts in relation to it, the suppo- 
sition of a globe of air over the spot will of course find no fit application (2877-)- We 
are first to suppose the sun far away and the atmosphere in a mean state as to tem- 
perature, and then consider the sun as present in the meridian of a given place ; and 
it is the degree of alteration in temperature and expansion of the air beneath and 
around the place of the sun, and the manner in which the change comes on and passes 
away, which concerns us. In relation to the surface of the earth, that alteration will 
be greatest somewhere beneath the sun, and will diminish in every direction around, 
becoming nearly nothing as to direct action at that part or circle of the earth where 
the sun’s rays are tangent. In relation to elevation, it is a question yet whether the 
effect is greatest in amount at the surface, diminishing upwards. As regards the atmo- 
sphere, it must of course end with it, though as respects space itself (2851.), a reser- 
vation thought may arise. With regard to any alteration occasioned by the sun’s 
influence in the opposite hemisphere, though there is none produced directly, yet in- 
directly there is that due to the falling of the temperature of the air, from the con- 
dition to which the sun, whilst above the horizon, had brought it. This change must 
be more tardy, irregular and disturbed, by local and other circumstances, than the 
opposite alterations produced by the direct influence of the luminary, and is that 
which occasions, by the hypothesis, the second maximum or minimum or other recur- 
ring night actions, made manifest by the needle in the hours when the sun is away. 
2919. The lines of force which issue from a magnet are, as it were, located and 
fixed by their roots in a way well understood experitnentally by those who have 
worked upon this subject. In the same manner the lines which issue from the earth 
more dr less suddenly, according to the amount of inclination, are held beneath by a 
force of location ; and because of the unchanging action of the earth in respect of 
atmospheric effect, are restrained more or less from alteration beneath during the 
changing action of the atmosphere. This fixation in the earth is a chief cause of 
certain peculiarities in the atmospheric phenomena as we observe them ; and is pro- 
ductive of that rotation of the line of force about the mean position which we have 
already eonsidered during the sun swing, and shall meet with again under the action 
of cold air. This condition of fixation at the lower parts of the lines of force occurs 
at every station where there is any dip at all, and gives for each the point of con- 
vergence round which the motion of the upper end of the needle takes place (2909. 
2932.). 
2920. So the atmosphere, under the influence of the sun, lies upon the earth 
altered most at the part beneath the luminary. It has received power to affeet the 
lines of magnetie force differently to the manner in which it affected them in the sun’s 
absence. It has become a great magnetic lens, able to refract the lines, and the 
manner in whieh it does so appears to be of the following nature. All the lines pass- 
ing through this heated and expanded air, surrounded by other air not so much 
