418 British Association. 



accordingly, may be coinpnted -when tlie other quantities are kno^vn. In 

 tLis manner, Prof. Haughton has deduced from the solar and lunar co- 

 efficients of the diurnal tide a mean depth of 5*12 miles — a result which 

 accords in a remarkable manner with that inferred from the ratio of tho 

 semi-diurnal co-efficients, as obtained by Laplace from the Brest observa- 

 tions. The subject, however, is far from being exhausted. The depth of 

 the sea, deduced from the solar and lunar tidal intervals, and from the 

 age of the lunar diurnal tide, is somewhat more than double of the fore- 

 going; and the consistency of the individual results is such as to indicate 

 that their wide difference from the former is not attributable to errors of 

 observation. Prof. Haughton throws out the conjecture that the de])th, 

 deduced from the tidal intcrvah and agcs^ corresponds to a different part 

 of the ocean fj'om that inferred from the heiglds. 



Terrestrial Magnetism. — The phenomena oi terrestrial magnetism pre- 

 sent many close analogies with those of the tides; and their study has 

 been, in a peculiar manner, connected wnth the labors of this Association. 

 To this body, and by the hands of its present General Secretary, were pre- 

 sented those reports on the distribution of the terrestrial magnetic force 

 which re-awalcened 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 philosophical sagacity of Ilerschel guided its 

 earlier career; and it w^as here again that the cultivators of the science 

 assembled, from every part of Europe, to deliberate about its future pro- 

 gress. It was natural, therefore, that the results obtained from such be- 

 ginnings should form a prominent topic in tlie 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 al- 

 ready 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 dag and a ymr, and w^hich, "therefore, were referred to the 

 sun as their source. To these periodical changes Dr. Lamont, of Munich, 

 added another ten years^ the diurnal 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 irregular intervals, 

 and which are characterized by a large deviation of the magnetic elements 

 from their normal state, and generally 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 

 Uie earth, and thus to indicate a cauvse operating upon the entire globe. 

 But, casual as they seem, these effects are found to be subject to laws of 

 their own. Prof. 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 period- 

 ical laws, depending upon the hour tit each station. The laws of this 

 penodicity have been ably woiled out by General Sabine in his discus- 

 sion of the results of the British Colonial Observatories ; and he has 

 added the important facts, that the same phenomena obser\^e also the two 



