MOUNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 



19 



the crater produces a whitish sky, as is a 2yi'iori probable. On one 

 occasion the definition was of high quality. It is Professor Hale's 

 opinion that the sky at Pike's Peak is considerably more pure ; and 

 this, again, is a priori likely to be the case. The forest-fires and the 

 dust from the plains to the east and from the South Park to the west 

 would seem to be the chief sources of solid particles in the surrounding 

 air, and all these taken together are not likely to produce as much 

 effect on the transparency of the sky as the smoke from Etna's crater. 



The favorable season on Etna is comparatively short, from the 

 middle of July till early October. The mean temperature of July at 

 Casa Inglese (about 9600 feet above sea) is about + 5° C. (41°.0 R), 

 the highest is about + 13° C. (55°.4 F.), and the lowest about — 1° C. 

 (30°. 2 F.). 



I have applied to my friend Professor Tacchini for his judgment of 

 the astronomical conditions on the summit of Etna, and the paragraph 

 which follows is extracted from his reply, dated January 23, 1896, to 

 my letter of inquiry. This verdict must be accepted as entirely 

 authoritative in all respects. 



" Quant a mon opinion sur les questions posees par vous, voila ma reponse : 



1°. Tlie sky is certainly markedlj' purer and more translucent on Etna than at 

 the sea-level. 



2°. The stars are markedly more steady on Etna than lower down. 



Mais, comme vous dites, suelement dans les meilleures conditions d'observation, 

 qui, dans les observatoires tres-eleves, ne sont pas aussi frequentes comme on 

 peut le croire." 



The Astronomical Observatory of Nice 



(on mount GROS, 1100 FEET ABOVE SEA). 



The observers at Nice have been too much occupied with making 

 valuable observations and discoveries to devote any considerable 

 amount of attention to investigating the conditions of the atmosphere 

 on Mount Grros, but their published volumes enable us to give a rough 

 estimate of the steadiness of the atmosphere there. In the double-star 

 measures of M. Perrotin with the 15-inch telescope, magnifying powers 

 of about 1000 diameters were habitually used. Each measure of a 

 star was marked a, 5, c, according as the images were good^ pretty-good^ 

 or moderately-good. 



I have had the curiosity to count the number of times each letter 

 occurs, as follows : 



from June, 1883, to August, 1886. 



