REPORT OF THE SECRETARY. 81 



nometer of M. Crova's well-kuovvii design, and constructed under M. Crova's personal 

 direction, was received at the OVjservatory. Tliis is a secondary type of instrument, 

 requiring standardization by comparison with some other radiation-measuring instru- 

 ment whose constants can be determined. Such an assumed standard was con- 

 structed here l)y inclosing the cylindrical bull) of a mercury thermometer in a flat 

 round box of very thin copper filled with mercury, nickeled on the back and sides, 

 blackened with platinum deposit in front, and situated in the center of a much larger 

 hollow, thick-walled wooden sphere coated with bright tin foil within, and provided 

 with a measured aperture opposite the front of the copper box, to which the solar 

 beam was led through adiaphragmed blackened tube. Such an instrument is essen- 

 tially the pyrheliometer of Pouillet, as advantageously modified by Tyndall by the 

 employment of mercury instead of water, but is rendered still more quick in its action 

 by the use of coj^per instead of the iron employed by Tyndall, and partakes somewhat 

 of the character of a perfect absorber or "black body," because inclosed in the tin- 

 foil coated hollow sphere. The water equivalent of the instrument was determined 

 by repeated calorimetric measurements. In use it has apparently worked perfectly, 

 responding so quickly to the heating of the solar rays that the rise of temperature in 

 the first 20 seconds after exposure is within two or three hundredths of a degree as 

 great as in the following 20-second intervals. 



Nevertheless there is a doubt as to the accuracy of this and, as I think, of all 

 instruments thus far used to measure the total solar radiation, from the fact that they 

 one and all receive the beam upon a front surface, which must necessarily get warmer 

 than the rear portion of the instrument, where the rise of temperature is observed, 

 and hence must lose a portion of the heat by a greater convection and radiation than 

 that which takes place after insolation has ceased. Thus a portion of the heat always 

 escapes measurement, and there is no ready way of knowing what its amount is. It 

 has been sought to devise here some type of standard pyrheliometer to which this 

 objection does not apply, and it is believed that such an instrument has been found, 

 although its construction is not yet comjilete. In principle it depends on receiving 

 the radiations within a hollow chamber or "black body" and carrying away the 

 heat by a continuous current of liquid, and the instrument, if successful, can be 

 employed as a continuous self-recording pyrheliometer. 



INIeanwhile repeated comparisons have made it sure that both the Crova actinom- 

 eterand the mercury pyrheliometer give readings j^roportional to the .solar radiation, 

 though there still remains some doubt as to the absolute magnitudes. Accordingly 

 one or both of these instruments have been read on days when the transmission of 

 the air has been determined, and these two kinds of data have been employed to 

 compute the solar constant of radiation or rate of receipt of solar radiation outside 

 the earth's atmosphere. 



THE SOLAR CONSTANT. 



This important quantity has been studied by the method you have devised and 

 described in the report of the Mount Whitney Expedition, and in a recent article in 

 the Astrophysical Journal for March, 1903. As employed here the method consists 

 in producing holographs of the solar spectrumj correcting the form of these for instru-' 

 mental absorption, and again for atmospheric absorption, and then multiplying the 

 rate of receipt of solar radiation at the earth's surface, as measured by the actinome- 

 ter, by the ratio of the areas included under the holographic curves, corrected for 

 atmospheric absorption and uncorrected, respectively. 



The work here has been more in the way of developing the inetliod of study and 

 obtaining experience in its use than in the expectation of measuring with certainty 

 the solar constant itself, for (as you have elsewhere observed), whereas it is in other 

 kinds of observation almost a (;ertainty that the mean of a series of observations is 

 more trustworthy than any single one, here a single observation made without inter- 



SM 1903 6 



