50 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



3. The bolometer developed by Langley (67), who was a pioneer 

 in investigations of the sun's energy, employs a blackened platinum 

 strip for the absorption of the rays, the thermal effect on this strip 

 being measured by its change in electrical resistance. 



4. The later developments along this line, described as pyrheli- 

 ometers are known, respectively, as the Angstrom (69), Callendar 

 (64) , Marvin (65) , and Smithsonian Institution Standard (50) . The 

 first three are constructed on the principle of electric resistance ther- 

 mometers, while the Smithsonian utilizes a mecurial thermometer. 

 The Callendar, it is understood, is distinguished by its automatic 

 arrangements for constantly recording the difference in resistance 

 between the absorbing and nonabsorbing plates or " grids." 



The technical differences between the several types of instruments 

 are so involved that a discussion of them can not be undertaken here. 

 They have to do largely with questions of efficiency in absorption 

 and measurement of the energy. In fact, the student of ecology 

 who plans to use any such instruments as these will be compelled 

 to make a most thorough study of the subject. Recent years have 

 seen so much attention given to it by physicists and meterologists 

 that, it may be said, the measurement of solar radiation is in a state 

 of flux. Bigelow (53) has recently questioned the adequacy of any 

 measurements made with pyrheliometers, declaring them useless, at 

 least for the determination of the solar constant. It will therefore 

 be the part of wisdom for biologists to stand aside until the physi- 

 cists have reached a more stable basis. 



In all of these instruments the auxiliary apparatus required is 

 considerable, except possibly with the Smithsonian. This is natu- 

 rally a deterrent to their use in the field, although the difficulties 

 may always be overcome when we are convinced of the usefulness 

 of the results. An instrument utilizing a mercurial thermometer 

 recommends itself for simplicity; yet, in view of the frequent 

 changes in the light intensity at any single point in the forest, the 

 equipment for continuous recording is not more than is needed for 

 satisfactory results. 



5. The thermometric sunshine recorder (TO), with electrical regis- 

 tering apparatus, is the equipment used at many Weather Bureau 

 stations for registering the duration of sunlight. This instrument 

 is extremely simple in design and operation, involving only the 

 movement of a column of mercury through a tube connecting a black- 

 ened and a transparent bulb of an air thermometer. When the 

 mercury reaches a certain height, as the result of air pressure in 

 the black bulb, it completes a circuit with the two platinum wires em- 

 bedded in the walls of the tube, and the current passing over this 

 circuit operates the registering mechanism. 



