110 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
careful investigation three promising sites had been chosen for further 
study and that in May 1948 a recording Eppley pyrheliometer was 
installed at each of these sites, namely, (1) Torreén, Coahuila, Méx- 
ico; (2) Mountain Pass, near Clark Mountain, California; and (3) 
Pohakuola, Hawaii. The three pyrheliometers were operated for a 
period of 1 year, ending June 1949. The resulting records indicate 
the uniformity and the quality of the sky for each day during the 
period. It is clear from the records that the best skies prevailed 
at the Clark Mountain location. The second-best site was Poha- 
kuola. This spot, 6,500 feet above sea level on the Island of Hawaii, 
yielded some records of unusually clear and uniform skies, but such 
skies were not the rule. At Clark Mountain, during the period 
June 8 to March 31, there were 171 days with skies sufficiently good 
for satisfactory observations, while at Table Mountain, Calif., during 
the same period actual observations were made on 135 days. From 
studies of these records and other sources, it appears that the Clark 
Mountain region is in general considerably drier and more free of 
haze and clouds than any other high-altitude location at present 
known in the northern hemisphere. 
In view of this, estimates were obtained of the cost of establishing 
a field station at an altitude of 6,500 feet on the south slope of Clark 
Mountain. Owing to the prevailing high prices for building materials 
and labor, the estimates proved to be in excess of available funds. 
It is hoped that sufficient funds may become available, but pending 
this the Observatory plans immediately to enlarge its facilities at 
Table Mountain sufficiently so that it will be possible to proceed 
without delay with the special experimental problems mentioned in 
last year’s report. 
Work at Washington.—W. H. Hoover, Chief of the Division, in 
addition to supervision of the work in progress, prepared data and 
tables which will help to simplify the computations in the field. In 
the past, to obtain the air mass (or length of path of the solar beam 
in the atmosphere) it has been necessary to plot carefully a series 
of theodolite readings against time, to read off desired altitude values, 
and finally to enter an air-mass-altitude table. With the aid of 
Hoover’s data, the observer, by reading the theodolite at specified 
intervals, may enter the tables directly to determine the air mass. 
This eliminates the tedious curve-plotting process. 
A new instrument, designed by Dr. John W. Evans, of the High 
Altitude Observatory of Harvard University, and described by him 
in the Journal of the Optical Society of America, December 1948, was 
kindly lent to the Astrophysical Observatory by Dr. Menzel of Har- 
vard University to test and to determine its adaptability to Smith- 
sonian work. The instrument is a photometer especially designed 
