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24 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
apparatus or to those unaccustomed to observation of the sun’s 
limb or disk that the true nature of the obstacle was pretty well 
understood before it was carefully investigated. It is now quite 
well settled that the “black drop” is due to bad eyes, imperfect 
apparatus, or the inexperience of the observer. With good eyes 
and proper apparatus a good observer never should see the black 
drop. When it is seen there is something wrong; it is a spurious 
phenomenon. 
_ One of the negatives was exhibited to the Society. 
In reply to a question by Mr. E. J. Farquaar, Mr. Eastman 
said the halo about Venus was believed to be due to the atmosphere 
of the planet. 
The next communication was by Mr. CLEVELAND ABBE on 
DETERMINING THE TEMPERATURE OF THE AIR. 
He stated that the question now to be considered is not where to 
place a thermometer so as to obtain the temperature most proper 
for the use of the meteorologist, but is rather the purely physical 
question of how to determine the temperature of the air at any 
given location. He described the methods and defects of the for- 
mer and present meteorological methods of exposure, viz: (1) Ther- 
mometers hung in the open air. (2) Those’ placed in shady loca- 
tions. (3) The Glaisher screen. (4) The Stevenson screen and 
the double louvre screens in general. (5) The double metallié cylin- 
drical shelters of Jelinek and Wild. (6) The silver thimble screen 
of Regnault. (7) The whirling thermometer of Saussure, Arago, 
Bravais, and the French observers (exhibiting Babinet’s arrange- 
ment as made by Casella.) (8) Joule’s method, depending on a 
balance in the temperature and density of two columns of the air. 
He then gave a description of the method devised by him in 
1865 and used for a short time at Poulkova; this consisted in con- 
structing a very perfect louvre screen, within which were established 
black bulb and bright or silvered bulb thermometers having very 
diverse coefficients of radiation and conduction. These thermom- 
eters were in air, not in vacuo, as this latter arrangement was proper 
only for the determination of the direct solar radiation, as in the 
Arago-Davy method, whereas in the present case the temperature 
of the air and the radiation from terrestrial objects were the special 
objects of study. 
