Lovering and Bond on Magnetic Observations at Cambridge. 31 
explain the modus operandi by which the sun’s rays affect the posi- 
tion of a magnetized bar. It is much to have the dependence of 
these changes upon solar time clearly established. 'The moments 
at which the maximum and minimum points of the diurnal curves 
occur oscillate between certain limits ; it is important, therefore, to 
have the diurnal curves for places removed many hours from each 
other in longitude, in order that a difference of time depending upon 
the constant difference of longitude may not be masked under the 
variation to which the time is subject at the same place. Now the 
Observatories of Gottingen and Cambridge differ in longitude 
5" 24’ 16”; it is plain, then, that if there be such a relation to 
the solar time of each place it must appear even in the curves of a 
single day. And so it is found by an examination of the Cambridge 
times and those published by Gauss for Gottingen and contiguous 
places that a difference equal to the difference of longitude exists in 
the mean times of the greatest eastern and western declination of 
the magnetic meridian. The extremes occur at nearly the same solar 
hours at each place, so that when we are observing our mini- 
mum in the morning the observer in those eastern longitudes is 
taking note of the western excursion of his bar; for at both places 
the interval from minimum to maximum is between five and six 
hours. The rest of the 24 hours is expended in the return of the 
bar back towards the morning position except in those cases 
where a secondary maximum and minimum occur; and if this une- 
qual time of passage over the same arc cannot be explained it is 
analogous to what is known to be true of the rise and fall of the 
temperature on the earth. The heating requires a shorter time than 
the cooling process; and whatever depends on the heating and cool- 
ing process will of course be subject to the same inequality. And 
in regard to all the magnetic changes which are attributed to the 
