OF THE MAGNETIC FORCE. 



51 



in the winter season ; in the annual curve there is barely a trace of it left. We 

 also recognize again the earlier occurrence of the maximum and minimum values 

 in winter and their later appearance in summer. If we examine the resulting curves 

 at Toronto 1 we find there the secondary morning fluctuation equally well marked 



(E). REGULAR SOLAR-DIURNAL VARIATION OF THE VERTICAL FORCE FOR WINTER, SUMMER, AND 



THE WHOLE YEAR. ' 

 (In parts of the force.) 



0" 



6 8 10 



14 16 18 20 22 24" 



Philadelphia mean time. 



in summer and winter ; and if we inquire into this feature for each year separately, 

 we find great irregularities between the hours 14 (Toronto astro'l time) and 22; in 

 1843 the secondary maximum and minimum is plainly developed, in 1844, and espe- 

 cially in 1845, it cannot be traced. Diagram (F) exhibits the curves for Philadel- 

 phia for each year. In 1841 and 1842 the curves are smooth, in 1843 the wave 

 appears well marked, in 1844 it is just perceptible. These apparent irregularities 

 are probably due to imperfections in our instruments ; on the other hand, if we take 

 the Philadelphia series, there may be a cyclic appearance and disappearance of 

 this wave. 



* VoL III of the Toronto Observations, Table LXVIII. 



