140 



MR BROUK ON THE RELATION OF THE VARIATIONS OF THE 



is no minimum visible near noon ; in February and November an inilexion occurs 

 there ; in March and October there is a marked tendency to a minimum at that 

 time ; tliere is a well-marlved minimum near noon in April and September ; a 

 strongly marked one about the same time in May and August ; and the principal 

 minimum occurs in June and July near mid-day. The same progression holds for 

 the maximum, which occurs near 20'', excepting that it never becomes the prin- 

 cipal maximum, though in June it differs very little from it. The inferior 

 maximum and minimum are best marked near the equinoxes. 



The following table will exhibit the transitions of the periods with more dis- 

 tinctness ; the negative sign ( - ) is placed before the hour of the principal mini- 

 mum, and the positive sign ( + ) before the hour of the principal maximum. 



Table of the Hours, Makerstoun mean solar time (astronomical reckoning), of the 

 Maxima and Minima of the vertical component of Magnetic Intensity for 

 1844. 



It will be observed that the hours of the first minimum and maximum in 

 this table occupy a year in completing their transitions, that the hour of the second 

 minimum in the table is constant, and that the hours of the second maximum, 

 and of the last minimum and maximum in the table, complete their transitions 

 in six months. 



9. Neglecting the inferior maximum and minimum, it will be remarked that 

 the period of duplication in the forms of the curves differs completely from that 

 for the horizontal component. For the latter, it is summer in which the diurnal 

 ciu-ve becomes single, and winter in which it becomes double. It is the secondary 

 or morning maximum of the horizontal component which becomes the principal 

 maximum in tvinter; it is the secoadary or noon minimum of the vertical com- 



* The principal minimum in the mean for the months of February and November actually occurs at 

 12'' 40™. This is due to an apparent irregularity in the month of November, that month being the only 

 one which does not shew the inferior maximum near midnight. 



