56 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVI.) 
2891. From a most important investigation by Colonel Sabine*, founded on the 
results of observations at Toronto and Hobarton, the facts appear to be that the mag- 
netic intensity is greater in both hemispheres in those months which are winter in the 
northern hemisphere, and summer in the southern. Similar results are greatly 
wanted for other localities, and would show whether the different disposition of 
land and sea has anything to do with the question, or whether the results at Toronto 
and Hobarton are true exponents of hemispherical effects. Assuming Toronto and 
Hobarton as being such exponents, the dip in both hemispheres is greater (i. e. greater 
north dip at Toronto and south dip at Hobarton) in those months which are winter 
in the northern, and summer in the southern hemisphere. Whether there is any 
annual variation of the dip or total force in the equatorial parts of the globe is very 
important to determine. It would be well worth while to take up a station for the 
express purpose ; the instruments are very simple, and the observations would require 
only a single observer. They are described in the paper referred to. Unfortunately 
such observations are not even made in Great Britain. 
2892. The manner in which the diurnal variation may be produced or affected by 
the action of the sun on our atmosphere as the earth revolves in its beams, has 
been already generally referred to. The whole portion of atmosphere exposed to the 
sun receives power to refract the lines of magnetic force which traverse it, and the 
whole of that which covers the darker hemisphere assumes an equally altered, but 
contrary state, relative to the mean condition of the air. It is as if the earth were 
inclosed within two enormous magnetic lenses competent to affect the direction of 
the lines of force passing through them. 
2893. I have already said that the action of the atmosphere thus affected might 
in some degree be compared at night time to that of an enormous, diffuse, and very 
feeble ordinary magnet, having the position that it would naturally take according to 
the line of dip, passing over us from east to west, and including us for the time within 
its influence : in the daytime the action would be like that of the similar journey, 
not of a corresponding magnet reversed in direction, but of a corresponding globe of 
diamagnetic matter (2821.). Assuming the maximum heat and cold to occur at 
midday and midnight, we might expect that the maximum effects would also occur 
near those periods as regards the variations of intensity (2824. 2866.) ; for, other 
things being the same, the central parts of the heated and cooled masses are those 
wliere the difference of intensity should be greatest. 
2894. It might be expected that this variation in the intensity would be greatest at 
those parts of the globe over which the sun passes vertically, or nearly so ; but that 
may depend upon two circumstances at least; first, whether the difference in the day 
* On the means adopted for determining the Absolute Values, Secular Change, and Annual Variation of the 
Magnetic Force, Philosophical Transactions, 1850, p. 201. 
