596 



and furnished in almost every instance with a numerous and well- 

 organized staff of observers, was the final triumph of a system which 

 had originated at Gottingen, and which has already sufficiently 

 pointed out the general laws, as well as the anomalies of magnetic 

 action, though unhappily it has hitherto left the physical causes 

 which give rise to them almost entirely untouched. 



We have already referred to the circumstances which suggested 

 the celebrated Memoir " On the Theory of the Earth's Magnetism," 

 which was published in 1839. There is, properly speaking, only 

 one known physical principle which can be assumed for its basis, 

 which is the variation of the magnetic forces, according to the in- 

 verse square of their distances. It is this principle which brings 







into operation a function, named by later writers the potential func- 

 tion, which had been already extensively used by La Place and 

 Poisson in some of their most difficult investigations arising out 

 of the theory of gravitation. The differential coefficients of this 

 function would express the coordinate components which determine 

 the direction and intensity of the earth's magnetism, and provided 

 they were known, they would assign the three elements which we 

 are in search of; but inasmuch as the law of the distribution of 

 magnetism within the earth is altogether unknown, so likewise is 

 the form of its potential function, and the process of deduction of 

 the conclusions which we are required to draw from it would thus 

 appear to be stopped at its origin. Yet there are some general 

 properties of the function itself, and some also which are dedu- 

 cible from the known conditions which it is required to satisfy, 

 which have enabled this great master of his art, with singular saga- 

 city and skill, to make even the dumb to speak, and to give re- 

 sponses which are of the highest philosophical import. 



Such is the clear conception, which he educes from it, of the 

 characteristic property of a magnetic pole, and the necessary con- 

 sequence resulting from it, that there can be only one northern and 

 one southern pole ; and the consequent effectual dissipation of the 

 conclusion which so many eminent philosophers had drawn from their 

 observations, that there were two northern, and by a natural in- 

 ference, therefore, two southern magnetic poles. Such also was the 

 remarkable proposition, that if the component of the horizontal 

 magnetic force directed towards the north was given for the whole 



