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LXXI. On the Height of the Aurora Bnrcalh from the Earth, 

 By Jamus Hoy, Esq. F.L.S. 



To Mr. Tilloch, 



Sir, — Upon reading in your useful Magazine for February 

 last, the Rev. Dr. I\Iaskelyne's " Plan for observing Fire-ball 

 Meteors," republished with Dr. Clarke's Additional Directions, 

 one cannot help regretting that these directions have been so 

 seldom put in practice. The reason, however, is obvious; — these 

 meteors appear so suddenly, move so rapidly, and disappear 

 again so quickly, that they are gone before one can pay such 

 attention to them as to determine any thing almost, except the 

 direction of their motion. The aurora borealis is another me- 

 teor, of which observations may be more easily made : but as 

 its rays or streams of light change their form and place, for the 

 most part, very quickly too, it is seldom that observers, at a di- 

 stance from one another, can make a simultaneous observation 

 upon one and the same point of the same ray, when the phae-. 

 nomenon gets the name of merry dancers. Sometimes, however, 

 it puts on the appearance of a luminous arch, nearly permanent 

 in the same place, or of such slow motion, that a difference of 

 five or even of ten minutes in the times of observing the alti- 

 tude of the top of the arch, or what stars it covers and makes 

 a near approach unto, &c. made by persons at a distance from 

 each other, would not mr.ch affect the result, in determining the 

 distance of the arch from the earth. 



Two luminous arches of this kind have been observed here 

 during the course of a twelvemonth; — the first on March 5, 

 ISI7. The exact time of its first appearance was not observed, 

 but at 8'' P.M. it was very brilliant, and of nearly .in equal 

 breadth throughout, which breadth filled the space between the 

 stars Castor and Pollux, tlie edges of it scarcely extending be- 

 yond them. 'I'racing it westward, its northern edge touched ///lie- 

 harun, and thence continued in a direct line to the horizon about 

 20" to the south of west. Its eastern end was as much to the 

 north of east. It narrovv^ed away to points at both ends. 



At S*" 32" its south edge passed about one degree above the 

 stars Regulus and Procyon; and to the westward its breadth 

 filled the space between Bellatrix and the foien»ost star (&) 

 of Orion's Belt. The zenith distance therefore of the highest 

 part of the arch was 45° 30' to the southward. Its light had 

 then become much fainter, and in five minutes afterwards it dis- 

 appeared altogether; a considerable time before the moon rose. 



This same arch v.-as mentioned in the Edinburgh newspapers 

 to have been observed at Glasgow: there at S'' 30'", it is said to 



have 



