68 Canadian Record of Science. 



" turn around the vertical ; " but I have not yet been able 

 to find particulars of an observation quite corresponding 

 to my own on this occasion. 



Either locality or the maxima and minima of periods of 

 frequency, may have a good deal to do with variations ; 

 but at the same time, I may say that the appearances in 

 this neighbourhood are fairly regular in behaviour, and 

 marked by typical weather conditions. The few excep- 

 tions that I have noticed seem to owe their peculiarity 

 to the abnormal development of some condition, rather 

 than to any distinct difference in the circumstances of the 

 display. I may indicate briefly why I am inclined to 

 attach importance to the observation under discussion. 



In the first place, the height of the aurora has been 

 computed to be at from 45 up to 500 miles above the 

 earth's surface. Lower estimates have been made, but 

 they are generally held to be in error. Professor Loomis 

 says : " That, although it is possible the aurora may some- 

 times descend nearly to the earth's surface, there is no 

 sufficient evidence to prove that the true polar light has 

 ever descended so low as the region of ordinary clouds." 

 Now, upon this occasion, while the light appeared to play 

 between cloud levels, twice very distinctly rays were seen 

 to gather more brightly, and leap up and down beneath 

 patches of cirro-cumulus cloud, which was more strongly 

 illuminated on the underside. Tbe general body of thin 

 cirrus clouds above seemed bathed in light, but in these 

 two instances, I am positive about the difference of the 

 illumination of the cirro-cumulus cloud being coincident 

 with the gathering of rays, about their position, which 

 apparently clung to them in a vertical beneath, and seemed 

 reaching to the heavy stratus cloud bank below. 



When you come to consider the practical difficulty of 

 getting any well defined edge or point, to take angular 

 measurements upon, much of the accuracy that we are 

 accustomed to attribute to theodolite observations disap- 



