240 



corresponds with the mean variation for the whole period ; the 

 nocturnal oscillation vanishing, and the course of the repre- 

 sentative curve, from 10 p.m. to 9 a.m., being in all nearly 

 a straight line. Thus the curves for the separate months ap- 

 pear to distribute themselves into two groups, depending upon 

 the position of the sun to the north or south of the equator ; 

 and it is remarkable that the transition from one to the other 

 system appears to take place abruptly, and almost per saltum. 



The whole range is nearly the same in each of the six 

 summer months. The maximum range occurs in April, and 

 its amount is 13'.3 ; this maximum is followed by a secondary 

 minimum in July, which is succeeded by a secondary maxi- 

 mum in August. There is a sudden change in the mag- 

 nitude of the range from February to March, and again 

 from October to November. The minimum range occurs in 

 December, and its amount is I'.O. 



The physical dependence of the phenomena of the changes 

 of the declination upon the sun is evident from the fact, that 

 they observe a diurnal and an annual period. In addition to 

 this fundamental fact, it has been long ago observed, with 

 respect to the diurnal change, that the time of the maximum 

 of westerly declination follows the sun's meridian passage at 

 a nearly constant interval, and that the morning and evening 

 minima are in like manner connected (although not so closely) 

 with the hours of sunrise and sunset ; and another point of 

 connexion between the cause and effect has been established 

 by the fact, long since observed, of the greater magnitude of 

 the range in summer than in winter. 



Dr. Lloyd proposes to show that the sun acts by means of 

 its heating power (as, in fact, is assumed both in the hypo- 

 thesis of Canton and that of Christie) ; and that the connexion 

 between the changes of declination and those of temperature 

 is more intimate than has been hitherto supposed. 



The force which produces the deviation of the magnet 

 from its mean position, at any moment of the day, is mea- 



i 



