104 abbot: solar constant of radiation 



ordinary thermometer is for measuring the temperature of the 

 air. 



Our first measurements of the sun's radiation as a whole were 

 made with the Crova alcohol actinometer, and in order to 

 standardize this instrument we constructed a modified Tyndall 

 pyrheliometer consisting of a copper box filled with mercury 

 and having a cylindric bulb thermometer inserted radially into 

 the box. Owing to the difficulty of keeping the small thread of 

 mercury at the proper point for reading purposes in the Crova 

 actinometer, we found it more desirable to develop the pyrheli- 

 ometer for our purpose. Soon a solid disk of copper with a radial 

 hole large enough to enclose the thermometer bulb was substi- 

 tuted for the box filled with mercury, the use of mercury being 

 limited to insuring a good heat connection between the bulb of 

 the thermometer and the copper of the disk. Some of these 

 copper disk pyrheliometers are still in use on Mount Wilson. 

 About 1909, however, the further improvement was introduced 

 of using silver in place of copper for the disk. A thin steel lin- 

 ing is provided for the hole where the thermometer is inserted, 

 so as to prevent the mercury from alloying with the silver. In 

 these silver disk instruments the thermometer stem, which is 

 introduced radially in the disk, is bent outside the chamber at 

 right angles so as to point towards the sun. The whole instru- 

 ment is mounted equatorially with a device for moving it by 

 hand to follow the sun from moment to moment. These disk 

 pyrheliometers, either of copper or silver, have now been in use 

 since 1906 with great satisfaction. Their constancy over long 

 periods of time leaves nothing to be desired, and the accuracy of 

 observation reaches a small fraction of 1 per cent. 



As the disk pyrheliometer is a secondary instrument, it was 

 necessary to develop a standard primary instrument to compare 

 it with. As early as 1904 experiments were begun to produce a 

 pyrheliometer based upon the hollow chamber ''black body" 

 type, with a flowing liquid to carry off the heat produced by 

 the absorption of the solar rays within such a chamber. After 

 numerous experiments, and long trial, the water-flow standard 

 pyrheliometer was fully developed in 1910. Later stifl, another 



