170 
Dr. Joule communicated the following extracts of letters 
referring to the auroral arch of the 20th of March last : — 
The Astronomer Royal to Alexander S. Herschel, Esq., 
Royal Observatory, Greenwich, 
1865, March 31. 
“ I have referred to our Magnetic Photograms on the 20th 
inst. The magnets were considerably disturbed for many 
hours before and after the appearance of the arch ; but the 
great disturbances of declination and hor. force were from 7h. 
to 8h. 45m., and those of vert, force nearly 45m. later. Of 
these you shall have copies when the time-scales are properly 
attached. There is scarcely a more important inquiry at the 
present time than that of the connexion between the indi- 
vidual phases of auroras, and the corresponding indiv.dual 
phases of magnetic disturbance. The first requisite is accu- 
rate clock time. By all means establish a reference to my 
Time Signals. 
I have seen many auroras, probably more than 20, fewer 
than 50. I am quite familiar with the ordinary appearances : 
low arch, grey or red streamers rising to or beyond the 
magnetic zenith, great red clouds, light flashing over limited 
spaces (the successive portions of space taking the light in 
successive portions of time) and others ; but I think I have 
seen the lofty well defined pale arch only twice. In some 
accounts of arches which I have read, two or three arches 
have followed in succession, all based in the magnetic E and 
W, and rising like a skipping-rope. As they approach 
very near the magnetic zenith, they exhibit a radiated 
structure And, finally, it appears that an arch really 
is a very long slender parallelepiped, its length in magnetic 
E and W consisting of beams in the direction of magnetic dip 
If you have not been possessed of this idea before, the 
possession of it may sharpen you in future observations or 
future inquiries.” 
