or Moon on Instruments at the Earth's Surface, 297 



which Jj/» = 9*, the magnetic moment of the whole is the sum 

 of the magnetic moments of its parts ; from which it follows that 

 the magnetic moments of similar bodies, if equally magnetized in 

 corresponding parts, are proportional to their volumes. There- 

 fore, as the action of the moon on our instruments varies as 



M 



— , we may substitute for the moon a hypothetical globe sub- 

 tending at our instruments the same angle as the moon, and 

 equally magnetized bulk for bulk. If, then, the moon be as 

 magnetic as the earth, its maximum effect will equal that of a 

 globe one metre in diameter, of materials as magnetic as the 

 earth, and placed at such a distance from the instrument as to 

 subtend an angle of 2043", which is the greatest apparent 

 diameter of the moon as seen from the surface of the earth. 



Now Gauss found the magnetic moment of a steel magnet 

 bar one pound in weight, referred to his absolute unit, to be 

 100,877,000, and he has shown f that the moment of the earth's 

 magnetism is equal to what would be produced by 7'831 such 

 bars placed parallel to one another in each cubic metre of its 

 volume. Hence the magnetic moment of a cubic metre mag- 

 netized in proportion to its bulk as much as the earth is 

 7-831 x 100,877,000 ; and multiplying this by '5236, the ratio 

 of the contents of a sphere to the cube of its diameter, we find 

 for the moment of the globe a metre in diameter, expressed in 

 Gauss's absolute units, 



M =0-5236 x 7-831 x 100,877,000. 



Again, 206264*8 being the number of seconds in radius unity, 



the distance of the globe, in order to subtend the same angle as 



i t -ni 206264-8 . . 



the moon when nearest, will be • .„ . metres, or (to express 



_ , -. ;•: 206264800 .... 



it in Gauss a unit or length) — — millimetres. 



Also, expressed in the same units, Gauss found the horizontal 

 intensity at Gottingen on the 19th July, 1834, H = 17748, and 

 the total intensity T = 4-7414. Therefore, finally, the maximum 

 deviations, expressed in seconds, which the moon, if as magnetic 

 bulk for bulk as the earth, could produce at Gottingen were 



* This is equivalent to requiring that the parts spoken of in the text be 

 formed by divisions so disposed as not to split any magnetic molecule in 

 such a way as would place the north magnetism it contains in one part and 

 the south magnetism in another. All fractures which can in practice be 

 effected, fulfil this condition. 



t See Gauss's Memoir "On the General Theory of Terrestrial Mag- 

 netism," translated in 'Taylor's Scientific Memoirs.' 



Phil. Mag. S. 4. Vol". 22. No. 1 17. Oct. 1 86 1 . X 



