"RESEARCH METHODS IN STUDY OF EOEEST 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. 



lNSTBTJMENTS AND APPROXIMATE C0STS. S 



Angstrom pyrhelionieters . 



Callendar pyrhelionieters $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 



CleniHiis photometer 7. 00 



Exposure-meters. (Photographic supply houses.) 1 to 5. 00 



Spectroscopes 20 to 100. 00 



Thermometric sunshine recorders: 



Sunshine recorder, electric, glass (not filled), 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-magnel registers — 



No. 1. For sunshine and rainfall (using Form No. 



loir, I',, 140.00 



No.2.«For wind velocity ami sunshine (using Form No. 



L015 O) L25.00 



So. i. For wind velocity, rainfall, and sunshine (using 



Form No. loir, E) i 10.00 



Quadruple register complete (for wind direction, wind 

 velocity, rainfall, and sunshine), with a year's supply 

 of blank Form* L017, pens, and ink 850.00 



* Tin v llOUld not I>' 



