RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 59 



evaporation of a gram of water requires a nearly constant a. .muni 

 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 

 lacking in moisture and the wind movement rapid, even an evap 

 rimeter 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 l>\ 

 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 

 instrument may be giving heat to the air and the other has heal 

 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 py rholiometers .... 



Callendar pyrheliometers 



Marvin pliyrheliometer — not on market. (U. S. Weather Bureau) .... 



Smithsonian pliyrheliometer (mercurial thermometer). (Smithsonian 



Institution) U :( ■ 



Sharpe-Millar photometer 100.00 



( Elements photometer 



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



Spectroscopes -'" '" 100 



Thermo nn trie sunshine recorders: 



Sunshine recorder, electric, glass (not filled), G. S. S. Xo. L2252. - 



Electrical sunshine recorded, complete 



Extra glass parts, mounted in brass socket, ready for attaching to 



support 



Registering instruments for use with thermometric recorders: 

 Two-magnet registers— 



Xo. 1. For sunshine and rainfall (using Form No. L015 B 

 Xo. 2. For wind velocity and sunshine (using Form No. 



1015-C) - l26J 



\o 4 For wind velocity, rainfall, and sunshine (using Form 



Xo. lOlo-E) 



Quadruple register complete (for wind direction, wind veloc- 

 ity, rainfall, and sunshine), with a years supply of blank 

 Forms 1017, pens, and ink ** 



8 These should not be taken as quotations. 



