Borealjs above the Sin face of the Earth. 425 



at 65°, and that at Warrington at 61°, the height comes out 

 150 or 160 miles, and its position vertical about Carlisle, which 

 is in opposition to the general tenor of the rest of the obser- 

 vations. 



As for the heights of the streamers or vertical beams seen 

 low in the north, we have no sufficient data for determining it. 

 But it is evident that the beams which were seen low at Edin- 

 burgh were the same as those seen still lower at Cockermouth, 

 Kendal, Lancaster, and Manchester, at which last place the 

 angle was about 10°, as my informant says. Now an object 

 elevated about 25° from the north at Edinburgh would appa- 

 rently be 10° or 12° at Manchester, if its real height were about 

 one hundred miles above the earth's surface. 



On the whole, I think it is fairly to be inferred that the 

 height of the arch could not differ much from one hundred 

 miles ; and that its breadth would be eight or nine miles, and 

 its visible length in an east and west direction, from any one 

 place, would be about 550 miles. 



Observations on other Aurorae. 



The height of a luminous arch calculated by the late Mr. 

 Cavendish, F.R.S. in the Phil. Trans, for 1790, is entitled to 

 notice. It was found to be betwixt fifty- two and seventy-one 

 miles. The observations, however, were made at too small 

 a distance from each other to admit of precision. A base of 

 at least forty or fifty miles seems necessary, where the object 

 to be measured is generally neither steady nor well-defined. 



The luminous arch seen at Keswick and Kendal by Mr. 

 Crosthwaite and myself, on February 15th, 1793, was calcu- 

 lated to be 150 miles high; but this was from a base of only 

 twenty-two miles. (See my Meteorological Observations and 

 Essays, page 69.) 



Dr. Thomson has given a brief history of the Aurora bo- 

 realis in the Annals of Philosophy for 181 4-, vol. iv. He has 

 copied a table from Bergman, being estimates of the heights 

 of about thirty aurorae observed during the last century, cal- 

 culated from observations made by different persons in various 

 places. According to these results, the aurorae would seem 

 to be of variable heights, from 130 to 1000 or more miles. 

 The places of observation are often unsuitably situated ; and the 

 data from which the calculations were made not being given, 

 I apprehend the great differences in the heights arise more 

 from defects in the observations than from real differences. 



In the same volume Mr. Longmire gives a description of a 

 luminous arch seen at Troutbeck near Kendal, on the 11th of 

 September, 1814. It was similar to that above described, and 



New Series. Vol. 4. No. 24. Dec. 1828. S I was 



