VOL. LXXX.] PHILOSOPHICAL TRANSACTIONS. 645 



X. On the Height of the Luminous Arch that was seen Feb. 23, 1784. By 

 Henry Cavendish, Esq., F. R. S., and A. S. p. 101. 



This arch was observed, at the same time, at Cambridge by Mr. Wollaston ; 

 at Kimbolton in Huntingdonshire, by the Rev. Mr. Hutchinson; and at Block- 

 ley near Campden in Gloucestershire, by Mr. Franklin; and is described in 

 letters from those gentlemen read to the r. s. in December 1786. See p. 627, &c. 

 of this volume. It has been remarked, that as the arches of the kind described 

 in these papers have usually but a very slow motion, their height above the sur- 

 face of the earth may readily be determined, provided they are observed about 

 the same time, at places sufficiently distant; and they seem to be the only me- 

 teors of the aurora kind whose height we have any means of ascertaining. The 

 places at which this phenomenon was seen are not so well suited for this purpose 

 as might at first be expected from their distance, because they lie too much in 

 the direction of the arch; they however seem sufficient to determine its height 

 within certain limits, and perhaps are as well adapted for it as any observations 

 we are likely to have of such phenomena. 



The latitude of Cambridge is 52° 12' 3Q"\ that of Kimbolton is said by Mr. 

 Hutchinson to be 52° 10', and according to the survey of Huntingdonshire, pub- 

 lished by Jefferies, is 52° 19' 50'^; so that we may suppose it to be 7 geogra- 

 phical miles north of Cambridge, and by the maps it seems to be about 1 8 such 

 miles west of it; and Blockley is by the map 12 geographical miles south and 72 

 west of Cambridge. At Cambridge the observations of its track seem to have 

 been made at about Q^ 15"" p. m. or 8^ sidereal time. At Kimbolton, allowing 

 for the difference of meridians, they could hardly have been made more than 5™ 

 sooner; and at Blockley they were most likely made nearly at the same times as 

 at Cambridge. 



At Blockley the arch passed about 7° south of the zenith ; but it is unneces- 

 sary to determine this point with precision. At Kimbolton it was found by a 

 quadrant to pase 11° to the south of it; and at Cambridge it was observed to 

 pass through $ and i Tauri, (3 Aurigae, 9 Ursae majoris. Cor Caroli, and Arcturus. 

 Now, if an arch was drawn through these stars, it must have appeared sensibly 

 waved to the eye; whereas Mr. Wollaston did not take notice of any crookedness 

 in this part of its course. It is most likely therefore, that the middle of the 

 arch must have passed to the south of (3 Aurigae, and to the north of 9 Ursae; 

 and if a circle is drawn through ^ Tauri, Arcturus, and a point 1° north of the 

 zenith, it will differ but little from a great circle, and will agree as well with the 

 positions of these stars as any regular line which can be drawn, and will pass 2\° 

 below p Auriga, and as much above 6 Ursae; which is not a greater difference 

 from observation than may well have taken place, considering how much care 



