Mr Potter on the Aurora Boreqlis. 213 



not render actual measurements the less necessary. And this 

 would become so easy for a low elevation of the meteor. 



Those who do not wish to take even so much trouble, may 

 satisfy themselves by viewing a stratus cloud which passes any 

 where near the zenith. If this cloud be really of pretty nearly 

 an equal breadth throughout its whole extent, it will, in con- 

 formity with the rules of perspective, appear much broader 

 near the zenith than in its more distant parts near the horizon. 

 Now, in a symmetrical regular arch of an aurora, no similar dif- 

 ference in breadth is ever perceptible, — a sufficient evidence 

 of their great distance from the observer. 



As to his second argument, it would at most show the me- 

 teor to be connected with the terrestrial system, and the view 

 first brought forward by Dr Halley, which has since been 

 completely established by Mr Dalton, that it is connected with 

 the earth's magnetism, is now universally admitted. Never- 

 theless, the apparent motion of the meteor, and particularly 

 its tendency to pass southward, are still subjects for inquiry 

 and discussion. 



It is one of the most singular features in the history of the 

 observations on the aurora, that its connection with the earth's 

 magnetism should have escaped the notice of such inquirers as 

 Mayer, Mairan, and Muschenbroek, who all refer the meteor 

 to the true north, though the magnetic declination was about 

 15,'i degrees west at London in 1731. And though some ob- 

 servations are recorded where it was noticed not to coincide 

 with the true north, it is not the less to the credit of Dr 

 Halley, that this great man, on the first aurora he saw, namely, 

 the famous one in March 3 716, which drew the attention of 

 so many learned men throughout all Europe to the subject, 

 perceived at once its great distance and height, ami argued 

 aUo on its connection with magnetic phenomena. 



Near the close of my former paper, I remarked that the 

 manner of finding the elements of an auroral arch from an ob- 

 servation of the points in which it cuts the horizon, and the 

 altitude of its highest point, was not the only way in which 

 we might consider the subject; assuming it still to be a sin.ill 

 circle round the magnetic axio, and in a plane perpendicular 

 to it ; but that we might substitute in place of the equation 



