346 



frequency in the two months. Upon the whole, it appears certain 

 that a minimum of actual as well as of visible frequency occurs in 

 summer ; a result quite in accordance with that for the amount of 

 magnetic disturbance, which accordance is sufficiently close to per- 

 mit us to complete it, by assuming that the number of aurorse is a 

 principal minimum in summer. 



It has been stated in the volume for 1844, p. 401, that this result 

 was long ago obtained by Mairan ; this statement, made chiefly on 

 the authority of Kaemtz and Hansteen, is not quite accurate. It is 

 true that Mairan''s numbers give a rough indication of the law, as 

 will be seen below ; but when it is remembered that his table in- 

 cludes all the observations (229) of which he could find a record for 

 upwards 1000 years, it will be evident, that the conclusion that a 

 gi'eater number of aurorse occurred at both equinoxes than at the 

 winter solstice would have been hasty ; this conclusion, however, is 

 not made by Mairan, and, though he has combined the numbers of 

 aurorae in a great variety of ways, he has made no combination ex- 

 hibiting this fact. It did not enter into the necessities of his theory 

 (that aurorse are the product of the solar atmosphere) to shew that a 

 greater number of aurorse happened in the northern hemisphere at 

 the vernal equinox than at the winter solstice ; he shews, indeed, that 

 the number for one equinox is, and, in accordance with his theory, 

 ought to be, greater than for the other. Some other philosopher has 

 the merit of first pointing out this fact. 



The following are the numbers of aurorse by Mairan (Traite Phy- 

 sique et Historique de I'Aurore Boreale, par M. de Mairan, 1733, 

 p. 199) ; by Ksemtz (Complete Course of Meteorology, translation 

 by Walker, p. 458) ; and by Hansteen (Mem. de I'Acad. Roy. de 

 Belgique, t. xx., p. 117). 



Mairan's numbers are probably included by Ksemtz ; a few of the 

 aurorse, included in M. Hansteen's list, are identical with those in 

 my own. 



