92 ANNUAL REPORT SMITHSONIAN INSTITUTION^ 1913. 



Toward the end of the fiscal year a vacuum bolometer was pre- 

 pared for use in continuing the experiments on the transmission of 

 very long wave rays through atmospheric water vapor. It is pro- 

 posed to push this work in the immediate future. 



6. THE CALIFORNIA EXPEDITION. 



A grant of money from the Hodgkins fund having been made by 

 the Institution to Mr. A. K. Angstrom for observations of nocturnal 

 radiation at different altitudes, several other lines of investigation 

 were arranged to be included in connection with these researches. 

 In the first place measurements were proposed on the total radiation 

 from the sky by day. For this purpose and with the aid of a small 

 grant from the Hodgkins fund Mr. Abbot devised and tested a 

 special sky-radiation apparatus. This instrument comprises two 

 blackened strips of metal, which are exposed successively at the 

 centers of two metal plates in such a way that the whole hemisphere 

 of the sl?y is free to shine on the exposed blackened strip, but nothing 

 can come from below the horizon toward the strips. Each strip is 

 at the center of a hemispherical glass inclosure, which serves the 

 purpose of preventing the exchange of rays of long-wave lengths 

 (associated with the temperature of such objects) between the black- 

 ened strip and the sky. Thus the apparatus serves to measure the 

 quantity of radiation, originally coming from the sun, which has 

 become diffusely scattered toward the horizontal surface by the 

 molecules and dust particles found in the atmosphere. 



Secondly, in order to determine the temperature and humidity 

 prevailing above the stations occupied by Mr. Angstrom's expedi- 

 tions, the Institution procured a large number of sounding balloons, 

 and arrangements were made with the Weather Bureau for flying 

 these balloons from Santa Catalina Island, carrying with each ascen- 

 sion self-recording -apparatus of the Weather Bureau for measuring 

 the temperature, pressure, and humidity of the air. Captive balloons 

 belonging to the Weather Bureau were also arranged to be sent up 

 at Lone Pine, Cal., and at Mount Whitney, Cal., while Mr. Angstrom 

 was occupying these two stations. 



As certain writers have expressed doubt whether measurements of 

 the solar constant of radiation made by Langley's method of high 

 and low observations with the spectrobolometer really furnish the 

 solar radiation values as they would be found outside our atmosphere, 

 it seemed desirable to check these results by observing at the highest 

 possible altitudes the actual intensity of the solar radiation. 

 For this purpose Mr. Abbott designed a form of pyrheliometer, 

 similar in principle to the" silver-disk pyrheliometer, but which 

 is automatic and self-recording, and can be attached to a sound- 

 ing balloon, and thus carried to very great heights. Five copies 



