RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 59 
evaporation of a gram of water requires a nearly constant amount 
of heat, varying according to well-known laws. The use of evapo- 
rimeters, however, has many complicating factors, principal among 
which is the air itself as a source of heat. If the atmosphere is lack- 
ing in moisture and the wind movement rapid, even an evaporimeter 
in the sun may be cooler than the air and consequently derive heat 
from the air. On the other hand, when the rate of evaporation is 
slow, the evaporimeter may be superheated, and some of the radiant 
energy absorbed will be dissipated into the air by radiation. 
The situation is by no means simplified by the use of a pair of 
evaporimeters, one of which is designed to absorb little of the radia- 
tion and the other much or all of it. In this combination, one instru- 
ment may be giving heat to the air, and the other has heat conducted 
to it. 
It therefore appears that evaporimeters may only give the broad- 
est possible comparison of light intensities, as, for example, when a 
number of similarly constructed instruments are exposed to similar 
atmospheric conditions. The latter, of course, are very likely to be 
modified by the same factors that modify the light. For these 
reasons, the method can not be recommended as an aid in the study 
of present problems. 
Instruments and Approximate Costs. 8 
Angstrom pyrheliometers 
Callendiir pyrheliometers $500. 00 
Marvin phyrheliometer — not on market. (U. S. Weather Bureau)- 
Smithsonian phyrheliometer (mercurial thermometer). (Smith- 
sonian Institution) 100. 00 
Sharpe-Millar photometer 100. 00 
Clements photometer 7. 00 
Exposure-meters. (Photographic supply houses.) 1 to 5. 00 
Spectroscopes 20 to 100. 00 
Therm am etric sunshine recorders: 
Sunshine recorder, electric, glass (not rilled), G. S. S. No. 
12252 3. 55 
Electrical sunshine recorded, complete 37.00 
Extra glass parts, mounted in brass socket, ready for at 
taching to support 23. 00 
Registering instruments for use with thermometric recorders: 
Two-magnet registers — 
No. 1. For sunshine and rainfall (using Form No. 
1015-B) 140. 00 
No. 2. For wind velocity and sunshine (using Form No. 
1015-C) 125.00 
No. 4. For wind velocity, rainfall, and sunshine (using 
Form No. 1015-E) 140.00 
Quadruple register complete (for wind direction, wind 
velocity, rainfall, and sunshine), with a year's supply 
of blank Forms 1017, pens, and ink 350. 00 
s These should not be taken as quotations. 
