18 mountain observatories in america and europe. 

 The Astronomical Observatory on Etna (9652 feet). 



The observatory on Etna was first proposed by Professor Tacchini, 

 in June, 1871, although, liis idea was not realized till 1881. As at 

 present organized, it is an annex of the observatory of Catania. The 

 35 cm. (13.8-iuch,) equatorial of the two observatories has a single 

 object-glass and two mountings, one at each station. During the 

 favorable season, July to October, the lens is mounted at the summit, 

 while it is employed at Catania for the remainder of the year. The 

 Etna station is reached by a drive of about eleven miles over a car- 

 riage road to Nicolosi, and from thence on horseback in six hours, 

 provided the trail is not obstructed by snow. 



Several important series of observations, having for their object the 

 determination of the relative advantages of high-and low-level observing 

 stations, have been carried out on Etna by Tacchini, Langley, Hale, 

 Eicco, and others. They need not be referred to in detail here. 

 The detailed report of Professor Hale and the general conclusions of 

 Professor Tacchini, which follow, seem to give all the information of 

 special value for our immediate purpose. 



Professor G. E. Hale spent some time on Etna in 1894 in an 

 attempt to photograph the solar corona in full sunshine. His notes on 

 the purity of the sky show that the bluei^iess of the sky increased 

 slightly from 1450 metres (4757 feet) up to the summit. The stars 

 were unsteady even at the zenith (July 8). 



On July 9 the sky was clear. A strong wind was blowing the smoke from 

 the great crater (which rose behind the observatory to an altitude of 3312 m., 

 10,866 feet) away (.from the direction of the sun. Half the island of Sicily was 

 dimly visible through a great brown bank of thick haze, the upper surface of which 

 seemed to be nearly on a level with us. The sun was seen (between clouds) to be 

 surrounded with a bright halo. In the afternoon the sky became much whiter. 



On July 10 the wind blew the smoke of the great crater over the sun, making ; 

 the sky very white. The image of the sun was rather better than at Catania, but 

 it became unsteady later. At 10 h. the sun was surrounded by a white halo, and 

 clouds of insects were noticed as at Pike's Peak in 1893. 



July 11. The sky was very white with a bright ring around the sun. The seeing 

 was excellent. 



July 12. The sky was very white, and there was a bank of haze above the level \ 

 of the observatory. The smoke from the crater was blown over the sun. The 

 sky seen from the summit of the great crater was bluer than when seen from the 

 observatory. The whole island was enveloped in haze. The seeing (at night) on \ 

 the moon, Saturn, and stars was magnificent. The images were almost perfectly i| 

 steady with the highest power. With the naked eye the twinkling of the stars fl 

 was hardly perceptible in stars higher than 80". I 



July 13. Sky cloudless and generally whitish, but the best seen since July 9. 

 Much dust. The definition on the sun was poor. i 



July 14. The whole eastern sky was white. Left the observatory this day. | 



These notes of Professor Hale's seem to show that the smoke from , 



