682 
SIR G. B. AIRY ON RESULTS DEDUCED FROM THE MEASURES 
Transactions. I have, therefore, decided on the following course. With the 
permission of the Astronomer Royal, I have adopted the three years 1868, 1864, 
and 1865, for partial exhibition of results. (Any other years would have answered 
equally well, for general exhibition.) For each of these years I have attached to 
this paper the curves for the months January, April, July, October, which suffice 
for showing generally the characteristic changes of magnetism for the several months. 
But some general account may be given, for which this is perhaps a suitable place. 
The form of the curves, and the position of the points on them corresponding 
to hours of solar time, leave no doubt that the diurnal inequality is due mainly— 
and, as far as I can judge, entirely—to the radiant heat of the sun ; and, it would 
seem, not to the sun’s heat on the earth generally, but to its heat on parts of the 
earth not very distant from the magnets. In the hot months of the year, the curve, 
though far from circular, surrounds the central point in a form which, as viewed 
from that central point, never crosses itself; and is, roughly speaking, usually 
symmetrical with regard to E. and W. But in the cold months, the space included in 
the curve is much smaller; in many cases, probably not more than one-sixth of what 
it is in the summer months; and the curve often crosses itself in the most bizarre 
fashion with irregular loops stretching out, three crossings in one curve occurring 
very frequently. In the summer months there is a certain degree of symmetry; 
but here is, constantly, a preponderance on the west side, which leads me to imagine 
that the magnetic effect of the radiant heat upon the sea is considerably greater 
than the effect on the land. 
To obtain some numerical basis for a report, which though exceedingly imperfect 
may convey some ideas on this wonderful subject, I have adopted the following 
course. I have confined myself to the months of June and July as probably the 
two hottest, and the months of December and January as probably the two coldest. 
For each of the curves applying to these months, I have laid down a system of 
rectangular co-ordinates, corresponding to the Greenwich meridian and to the line 
at right angles to the meridian (or the geographical E. and W.). The extreme 
north ordinate and the extreme south ordinate are measured, and their sum is taken, 
and interpreted by a scale of measure formed in accordance with the theory of the 
instruments; and this interpreted sum forms the “Range of Meridian Force” 
in terms of the whole Meridian Force. In the same manner, the “Range of Trans¬ 
versal Force” is measured. As the time of each of the two-hourly or hourly records 
