Vol. 6, 1920 
ASTRONOMY: C. G. ABBOT 
83 
or three times in the day, they are quite inadequate to enable the investi- 
gator interested in estabHshing solar radiation stations to select the regions 
of the earth best meeting his requirements of cloudlessness. It would, 
therefore, be a very valuable investigation if some suitable autographic 
method of measuring cloudiness could be devised simple enough to be em- 
ployed at numerous stations reported to be cloudless. 
The whole subject of the cloudiness of the earth is very important. 
Recent measurements of Aldrich show that a cloud surface reflects about 
78% of the solar radiation incident upon it. As the cloudiness of the earth 
is reputed to be about 50%, it is obvious what a large part clouds play in 
determining the temperature. They are effective not only in cutting off 
incoming radiation from the sun but also outgoing radiation from the earth. 
If the simple autographic apparatus above mentioned were available, it 
should be employed at numerous stations in all parts of the world and if 
possible on the ocean. 
The measurement of solar radiation at several stations on the earth's 
surface by means of the pyrheliometer has occupied observers in many 
countries. Several series of observations have been carried on for 
many years by the U. S. Weather Bureau, and are published in excellent 
form in the Monthly Weather Review. Many copies of the Angstrom 
pyrheliometer and of the Smithsonian silver disc pyrheliometer, whose 
constants are both so well known as to form together a homogeneous system 
of pyrheliometry, have been sent out to many quarters of the world. 
Only a few series of regular observations are at the present time being 
conducted with them, and it is greatly to be hoped that a large number 
of such series may be conducted regularly in the future. The method of 
reducing the results and publishing them can hardly be improved over 
that which is followed by the United States Weather Bureau, and that 
may well be taken as a model. 
The study of the outgoing radiation from the earth has lately been 
carried on by several observers, notably by Mr. Fowle, who has investi- 
gated the effects of water vapor in long atmospheric columns in diminishing 
the rays of long wave-length such as the earth sends out. His investiga- 
tions extend only to a wave-length of 17 microns because no suitable optical 
media transparent to waves of longer wave-length are available. It ap- 
pears that potassium iodide may be such a medium and it is hoped shortly 
to produce large crystals of the salt suitable for making prisms for the 
study of long wave-lengths of terrestrial radiation. 
In the meantime, valuable information has been obtained on terrestrial 
and atmospheric long wave-length radiation by Dr. A. K. Angstrom 
by the use of the pyrgeometer or nocturnal radiation instrument devised 
by his father. An instrument applicable for the same purpose has also 
been devised by Messrs. Abbot and Aldrich of the Smithsonian Institution 
and is called by them the pyranometer. Neither instrument is entirely 
