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 tliermometric 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. 
