2 Mr, Boner on the [July, 



the opinion of those, who attribute the variation to the changes 

 produced in the iron mines by the continual excavations made 

 therein by men's hands. 



The variation of the compass being evidently the effect of 

 attraction, would it not be more philosophical to look for its 

 cause in that universal power, by which the whole planetary 

 system is acknowledged to be governed. Few persons deny 

 tne influence of the sun and the moon in raising the waters of 

 the sea; and I can see no incongruity in the idea, that the varia- 

 tion is an effect of the same, or at least of a similar cause. 



Repeated observations prove that at the same place the varia- 

 tion is different at different hours of the day ; and Mr. Canton 

 affirms, that in 574 observations he found the variation regularly 

 increasing westward from about eight or nine in the morning 

 till one or two in the afternoon; when the needle became 

 stationaiy for some time, after which the absolute variation west- 

 ward was decreasing, and the needle came back again to its 

 former situation or near it in the night, or by the next morning. 



1 ask now whether there is any thing more like the periodical 

 rise and fall of the waters of the sea, which, as every body 

 knows, happen twice every four and twenty hours, and consi- 

 dering that the diurnal east and west variations are very nearly 

 at the same distance from noon, I have little doubt but that if 

 the observations were made as regularly during the night, we 

 should discover the same changes before and after midnight, 

 when the sun is in the opposite meridian, and thus find two 

 magnetic tides, if I may use the expression, as we have two sea 

 tides eveiy day. It appears therefore not at all improbable to 

 me, that the periodical change of the variation should be regu- 

 lated by the situation of the heavenly bodies, and the whole 

 revolution of the magnetic pole be performed within the space 

 of 532 years, a period derived from the multiplication of the 

 numbers 19 and 28 ; that is, of the lunar and solar cycles 

 together. 



J5ut what confirms me still more in the idea that the principal 

 cause of variation resides in the region of the planets, is, that by 

 the first trial which I made to discover the place of the mag- 

 netic pole by means of the dip and variation observed in 1812, 

 I found the annual progress of the magnetic pole in direct pro- 

 portion as the annual progress of the nodes of Venus to the 

 annual precession of the nodes of the earth; that is, as 31'^ : 

 50'''*25 : : annual progress of the magnetic pole : to one degree 

 or 60 minutes ; which gives the annual progress 37' 00'^ 53''''73. 

 And from the dip and variation observed in London in the year 

 1812, I find the annual progress of the magnetic pole, as it will 

 be seen hereafter, equal to 37' 63^' ll^''-22, differing only 

 52// 17^^'-49 from the former ; and if to the first 37' 00'' 53-'"73 

 v/^ add the annual precession of the equinoxes 50^" seconds, 



