206 ON MAGNETISM. 



though there is a sensible difference (as has been stated) 

 in the forms of the curves for different years, yet those 

 characteristics of the curves upon which the eye rests 

 as marking its most striking peculiarities are repro- 

 duced with accurate resemblance in all. 



The curves for the different months have a marked 

 difference. In the summer months, the curves are 

 larger and more nearly round, and the small appendage 

 about the early morning-hours (15 h , 16 h , &c.) is less 

 strongly marked: in the winter, the curves generally 

 are smaller, and the morning-appendage is more im- 

 portant. 



If, instead of using solar hours to define the times 

 of measure of our ordinates, lunar hours (reckoned from 

 the time of moon's transit over the meridian) are 

 employed, we obtain a remarkable result. The solar 

 diurnal inequalities disappear entirely from Jbhe mean; 

 and we find that there is a true lunar tide of magnet- 

 ism, occurring twice in the lunar day, and shewing 

 magnetic attraction backward and forward in the line 

 from the Red Sea to Hudson's Bay. These forces are 

 however considerably less than those which follow the 

 law of solar hours. The mean diurnal solar inequality 



may be stated as about + -^ of horizontal force: the 



600 



lunar is about + 



- 12000 ' 



