I9II-] ABBOT— SOLAR CONSTANT OF RADIATION. 237 



ment.* Finally we have ceased to regard our instrument as giving 

 more than relative measurements. It is only a secondary pyr- 

 heliometer for convenient use. We standardize its readings hy com- 

 parison with an absolute pyrheliometer of another kind. 



No known substance absorbs radiation perfectly at a single en- 

 counter. Kirchhoff showed, fifty years ago. that a hollow chamber 

 must absorb perfectly, because of the opportunity for an infinite 

 number of absorption encounters within it. W. A. Michelson, in 

 1894, invented a standard pyrheliometer including a hollow chamber 

 with a narrow opening for the admission of rays. The walls of the 

 chamber were bathed by a mixture of ice and water, and the heating 

 effect of the solar rays was measured by the amount of ice melted, 

 which was determined by noting the expansion in volume of the 

 mixture of ice and water. 



Nearly ten years later, being ignorant of Alichelson's pyr- 

 heliometer (which was described in the Russian language), it 

 occurred to me also to employ a hollow receiving chamber. I pro- 

 posed to measure the solar heating produced in it by bathing its 

 walls with flowing water, and determining the rate of flow and rise 

 of temperature of the water. After experiments lasting inter- 

 mittently from 1904 to 1910, I am now satisfied that this device 

 has proved successful, and that we have truly an absolute standard 

 pyrheliometer. With the aid of my colleagues, Mr. Aldrich and 

 Mr. Fowle, two of these water-flow pyrheliometers were carefully 

 tested last year.^ Not only did they agree in measurements of solar 

 radiation, but test cjuantities of heat introduced electrically within 

 the absorbing chambers were accurately recorded by the methods 

 ordinarily used to measure solar heating. We believe of the abso- 

 lute water-flow pyrheliometer, it gives the intensity of solar radia- 

 tion at the earth's surface in calories per square centimeter per 

 minute within a probable error of 0.2 per cent. For convenience 

 we make our flaily observations with secondary silver-disk pyr- 

 heliometers, which have been standardized against the absolute 

 water-flow pyrheliometer. 



* See Abbot, "The Silver Disk Pyrheliometer," Sniitlisoii. Misc. Coll., 

 Vol. 56, No. 19, 191 1. 



° See Abbot and Aldrich, Astrophys. Journal, Vol. XXXIII., 125, 1911. 



