Scientific Lectures. 183 



ment that the force decreases, in its action, as the squares of the 

 distances we recede to, from the magnetic poles ; * that is, at a dis- 

 tance of ten feet, say the force is one ; then at twenty feet it will be 

 one-fourth of what it was at ten feet ; and at thirty feet it will be 

 one-ninth, and so on. Hence, the strength of our large magnet is to 

 the strength of the earth's magnetism as the distance of the coil 

 from the poles of the electro-magnet is the distance of the coil from 

 the poles of the earth. By such a rough measure we will gain some 

 idea of the enormous magnetic effect with which the earth influences 

 surrounding space. Gauss, the illustrious German astronomer and 

 natural philosopher, calculated this quantitiy, and taking as the unit 

 of his measure a magnet of the size of this I hold in my hand — 

 which is fourteen inches long, one inch wide and one-fourth inch 

 thick, and weighs one pound — he found that the earth's magnetism 

 is equal to 8,464,000,000,000,000,000,000 such magnets. That is, to 

 this number of the hardest steel magnets, each weighing one pound, 

 and made as strongly magnetic as possible. 



But if the earth contains all this magnetism, from which we have 

 a current of electricity, is it not likely that some of our previous 

 experiments with the electro-magnet were vitiated by this earth-cur- 

 rent coming into our coil ? This supposition explains why I always 

 rotated the coil around a vertical axis ; for, look, when I do so I do 

 not deflect the galvanometer needle, for I do not cut the magnetic 

 lines in such a manner as to develop a current that can flow out of 

 the coil, because the current in one-half of the circle of the coil is 

 opposed to that generated in the other half, and they therefore neu- 

 tralize each other. 



In the experiments you have just witnessed, the electric current 

 from the earth's magnetism produced a deflection of only 15° to 20° 

 in the lantern needle ; but if I connect this coil with the delicate gal- 

 vanometer in my laboratory, I cannot lift one side of it six inches 

 above the table, thus, without giving the needle a deflection of 20°, 

 and if I place the coil in a vertical plane, and tilt eight inches toward 

 the horizon, the needle swings through a quarter of its circle. With 

 this coil and sensitive galvanometer, the difficulty is not to produce 

 these currrents from the earth, but it is to prevent them from contin- 

 ually mingling with the other effects of your experiments. 



♦Tobias Mayer— in an important unpublished paper, read before the Roy. Soe. Gottingen, refer- 

 red to by Lambert — showed, in 1750, that the law of variation of the magnetic attraction corre- 

 sponds to that of gravity. Lambert, Hist. Acad. Roy. Sci., Berlin, 1776 ; " a memoir," says Robinson, 

 which, "would have done credit to Newton himself." Coulombe, Roy. Acad. Sci., Paris, 1786 

 and 1787. 



