424- Mr. R. Potter on a brilliant Arch 



hers 73° N. latitude and 100° W. longitude as the position of 

 the magnetic pole, which, from the observations made during 

 Captain Parry's voyage, is probably not widely wrong, we 

 find the magnetic polar distance, or magnetic co-latitude, of 

 Edinburgh to be 39° 23', that of Armagh 39° 55', and that 

 of Athboy 40° 21'. Now at all places situated on the same 

 parallel of magnetic latitude, it is clear that an arch following 

 the direction just mentioned would appear at the same alti- 

 tude at its highest point, which enables us to reduce ob- 

 servations anywhere taken to a common magnetic meridian; 

 and Armagh being only 32' of a degree more magnetically 

 southerly than Edinburgh, the arch ought to have been seen 

 at the former place with a little greater altitude from the 

 S.S.E. horizon than at the latter, which we find to have been 

 the case, as the following shows: — 



At Armagh, and 9 h 10 m 45 s Greenwich time, the arch was 

 bisected by y Leonis, then not far from the magnetic meri- 

 dian. 



At Edinburgh, and 9 h 7 m 41 s Greenwich time, the arch 

 passed over a Leonis, and allowing still for the southward 

 motion of the arch during the remaining 3 ra 4 s difference of 

 time, we see that the arch had a considerably less apparent 

 altitude at Edinburgh than at Armagh. 



Though under so many disadvantageous circumstances, — 

 of a base line of only 32 geographical miles when the places 

 were so far distant, the quick motion of the arch, and the 

 want of simultaneousness in the times of observing, — yet the 

 notes taken by these accurate observers enable us to make a 

 computation for the height of the meteor which agrees re- 

 markably closely with former determinations, and is quite 

 within the limits furnished by observations taken under much 

 more favourable circumstances. 



The difference of magnetic polar distance of the two places 

 of observation is 32 geographical, or 36*84 English miles 

 nearly; and the apparent altitude at Edinburgh for the mid- 

 dle of the arch, about 43° £9'; and at Armagh 52° 25' at 

 9 h 10 ra 45 s Greenwich time. These altitudes require still to 

 be corrected for the inclination of the horizons of the two 

 places, which requires lb"' to be added to the former, and sub- 

 tracted from the latter of these angles. From these data we 

 find the arch to have been 178*43 English miles from Armagh, 

 and 142*84 miles above the surface of the earth. 



The calculation which I first made by using Lord Darnley's 

 observation, must not, I find, be insisted upon, on account 

 that the time of it is not sufficiently indicated in the abstract 

 in the " Proceedings" of the Royal Society to enable us to 



