li REPORT—1857. 
and lunar ¢idal intervals, and from the age of the lunar diurnal tide, is some- 
what more than double of the foregoing ; and the consistency of the indi- 
vidual results is such as to indicate, that their wide difference from the former 
is not attributable to errors of observation. Professor Haughton throws out 
the conjecture that the depth, deduced from the tidal intervals and ages, corre- 
sponds to a different part of the ocean from that inferred from the heaghts. 
The phenomena of Terrestrial Magnetism present many close analogies 
with those of the tides; and their study has been, in a peculiar manner, con- 
nected with the labours of this Association. To this body, and by the hands 
of its present General Secretary, were presented those Reports on the dis- 
tribution of the Terrestrial Magnetic Force which reawakened the attention 
of the scientific world to the subject. It was in the Committee-rooms of 
this Association that the first step was taken towards that great magnetic 
organization which has borne so much fruit ;—it was here that the philoso- 
phical sagacity of Herschel guided its earlier career ;—and it was here again 
that the cultivators of the science assembled, from every part of Europe, to 
deliberate about its future progress. It was natural, therefore, that the re- 
sults obtained from such beginnings should form a prominent topic in the 
addresses which have been annually delivered from this Chair; and the same 
circumstances will plead my excuse, if I now revert to some of them which 
have been already touched upon by my predecessors. 
It has been long known that the elements of the Earth’s magnetic force 
were subject to certain regular and recurring changes, whose periods were, 
respectively, a day and a year, and which, therefore, were referred to the Sun 
as their source. To these periodical changes Dr. Lamont, of Munich, added 
another of ¢en years, the diyrnal range of the magnetic declination having 
been found to pass from a maximum to a minimum, and back again, in 
about that time. 
But besides these slow and regular changes, there are others of a different 
class, which recur at ¢rregular intervals, and which are characterized by a 
large deviation of the magnetic elements from their normal state, and gene- 
rally also by rapid fluctuation and change. These phenomena, called by 
Humboldt “magnetic storms,” have been observed to occur simultaneously 
in the most distant parts of the earth, and therefore indicate the operation 
of causes affecting the entire globe. But, casual as they seem, they are 
found to be subject to laws of their own. Professor Kreil was the first to 
discover that, at a given place, they recurred more frequently at certain 
hours of the day than at others; and that consequently, in their mean effects, 
they were subject to periodical laws, depending upon the howr at each station. 
The laws of this periodicity have been ably worked out by General Sabine, 
in his discussion of the results of the British Colonial Observatories ; and 
he has added the important facts, that the same phenomena observe also the 
two other periods already noticed, namely the annual and the decennial 
periods. He has further arrived at the very remarkable result, that the de- 
