REPORT OF COMMITTEE ON OBSERVATORIES 1 55 



[Fro?n Professor A . Riccb, Director of the Royal Observatories of Catania 



and Etna, Sicily.] 



[Translation.] 



February 28, 1903. 



With regard to the project under consideration by the Committee 

 of the Carnegie Institution, of which you are a member, it is certain 

 that if a station at the summit of a very high mountain is needed 

 for the study of the solar radiation, another station will also be re- 

 quired which may be conducted without encountering the difficulties 

 which are unavoidable at elevated stations ; among others, the fre- 

 quency of days when the mountain is enveloped with clouds due to 

 the condensation which it produces. Etna, for example, as seen 

 from Catania, is enveloped with clouds 167 days in the year, on an 

 average. 



The other station should not be placed at the foot of the moun- 

 tain, since it would also experience the effect of condensations caused 

 by the mountain. At Catania the clouds cover on an average only 

 39 per cent of the sky (Palermo, 46 per cent), but this amount of 

 cloudiness would be still smaller if it were not for Etna, since the 

 clouds appear more often on the side toward Etna (north) than else- 

 where. Furthermore, the station at the base of the mountain would 

 not have an entirely free horizon in one direction. 



There should also be a third station on a very extensive high 

 plateau on a small island, in order to have a very homogeneous 

 atmosphere in perfect equilibrium, and to avoid disturbances of the 

 images caused by ascending currents ; these are very pronounced 

 during the hotter part of the day on Etna, and carry up with them 

 visible vapors, which frequently hide the sun, and invisible vapors 

 which produce remarkable absorption of the solar radiations. On 

 Etna the curves of heat received by the Arago actinometer have this 

 form [figure not reproduced here] . This may be explained by the 

 absorbing action of the ascending vapors during the hours of greatest 

 heat. At Catania the curves have the regular form, rising with the 

 altitude of the sun. 



At Catania, as at Palermo, the images of the sun are best in the 

 early morning ; ordinarily during the hot seasons they are bad at 

 10 o'clock ; they become better again before sunset. 



