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Progress of Foreign Science. 



professor made during the course of the year, he has calculated 

 the mean monthly intensity of the magnetic force. The fol- 

 lowing is a table of the results : 



From these observations it follows ; first, that these is a daily 

 variation in the strength of the magnetic power; the minimum 

 occurring between 10 to 11 A.M, the maximum between 4 and 

 5 P.M. Second, that the magnetic power is subject also to a 

 yearly variation in its magnitude, as appears from the mean of 

 each month ; its amount is manifestly greater in winter, while 

 the sun is in perigee, than in the opposite season ; in December 

 and June the greatest monthly variations also take place ; 

 and the minimum variations occur in the two months of spring 

 and autumn, when the sun is at his mean distance from the 

 earth. The greatest daily variation of the magnetic power is in 

 summer ; the least in winter. The maximum difference of the 

 annual variation is 0.0359. 



If we denote by F the total magnetic power, by H that 

 portion of this power which acts on a horizontal needle, and by 

 n the magnetic inclination, then H e= F+' cos. n. Hence H 

 may be variable, when the whole magnetic force F is constant, 

 provided the magnetic inclination be variable. But M. Hansteen 

 imagines, from researches which he made in 1820, with a dip- 

 ping needle by DoUond, that he found the inclination to be in 

 summer about 15' greater than in winter ; and from 4' to 5' greater 

 in the forenoon than in the afternoon ; which would accord 

 with the foregoing observations *. 



* On this subject M. Arago judiciously observes, that from Mr. Gilpin's 

 observations, whose accuracy is acknowledged by philosophers, there 

 appears to be no appreciable diurnal variation in the dip, and its monthly 



