ASTRONOMY: ABBOT AND ALDRICH 
333 
ingly rapid rates of attenuation, remains to be determined. With the 
help of Dr. H. L. Howes, to whose assistance throughout this investi- 
gation the author is greatly indebted, this question is now under inves- 
tigation. 
1 Lenard, Ann. Physik, ser. 4, 31, 641 (1910). 
2 E. L. Nichols, Paper presented before the Amer. Philos. Soc, April, 1916. 
3 Nichols and Howes, Paper presented before the Amer. Physic. Soc, April, 1916. 
^ Nichols and Howes, Physic. Rev., ser. 2, 4, 19 (1914). 
' Kennard, Physic. Rev., ser. 2, 4, 278. 
THE PYRANOMETER: AN INSTRUMENT FOR MEASURING 
SKY RADIATION 
By C. G. Abbot and L. B. Aldrich 
ASTROPHYSICAL OBSERVATORY. SMITHSONIAN INSTITUTION 
Read before the Academy, April 1 8, 1 9 1 6. Received April 30, 1 9 1 6 
This instrument, as its name (from the Greek irvp, fire, ava, up, 
fierpov, a measure) indicates, is intended to measure the heat equiva- 
lent of radiation received from or going out toward the complete hemi- 
sphere above the plane of the measuring surface. We have devised two 
satisfactory types of the instrument, both derived in principle from the 
electrical compensation radiation instruments of the late K. Angstrom. 
The full description of the instruments and tests of them will be found 
in a paper now being published in the Smithsonian Institution Miscel- 
laneous Collections. The instruments are adapted to measure direct 
solar radiation, the total radiation of the sun and sky combined, that 
of the sky alone, and nocturnal radiation. It is possible to employ 
screens of selective transmission and thus to limit the measurements to 
selected spectrum regions. The instruments are of primary standard 
type, but have been compared with the standardized pyrheliometers of 
the Smithsonian Institution and found accordant. No auxihary appa- 
ratus other than that employed with the Angstrom pyrheliometer is 
required, and the observations are easy to make. 
The simpler form of pyranometer comprises a single blackened man- 
ganin strip, 3 mm. wide, 6 mm. long, placed centrally in the plane of the 
upper surface of a nickel-plated copper disk 75 mm. in diameter. Cop- 
per blocks insulated from the rest of the disk, but continuous with it in 
surface, serve to connect the insulated manganin strip with an electric 
heating current of adjustable strength. A sensitive thermo-electric 
couple fastened by means of thin waxed paper to the rear surface of the 
manganin strip, and embedded at the other end in a recess of the copper 
disk serves to indicate changes of temperature of the strip. Concen- 
