NO. 19 THE SILVER DISK PYRIIKEIOMETER — AP.BOT 3 



units. Pouillet, it is true, deiermined the dimensions of his pyr- 

 heHometer, and from them reduced his results approximately to 

 calories per square centimeter per minute. But owing to several 

 uncertainties not necessary to mention here, it is not practicable to 

 achieve sufficient accuracy in such standardizations of pyrheliometers 

 of this type. They must therefore be regarded as secondary in- 

 struments, useful only for relative readings, unless standardized 

 by comparison with true standard pyrheliometers. Such standardi- 

 zations have been made at the Astrophysical Observiitory.^ 



Regarding the silver disk pyrheliometers as secondary instru- 

 ments, an abridged method of reading is possible, which materially 

 reduces the labor of observation and reduction. In ordinary calor- 

 imetry the thermometer is read as frequently as possible, often at 

 10 or 20 second intervals, in order that a graphical representation of 

 the whole march of temperatures may be made. In this way the 

 most exact determination is possible of the rate of rise of tempera- 

 ture due to the source of heat itself, irrespective of cooling or warm- 

 ing due to the surroundings. In the use of the pyrheliometer as a 

 secondary instrument the true value of the rate of rise of tempera- 

 ture due to solar heating is not material. If a simplified method of 

 observation can furnish results which are under all circumstances 

 greater or smaller by a constant fraction than the true rate of rise, 

 such results are equally as valuable as the true ones would be. For 

 the standardization constant of the instrument corrects such errors. 

 By numerous experiments it has been shown that the short method 

 of reading the instrument to be described below satisfies the condi- 

 tion just explained, well within the error of the observations, hence 

 it has been adopted. 



In order to promote pyrheliometric measurements of the solar 

 radiation in other parts of the world, with instruments whose indi- 

 cations are quite comparable. Secretary Walcott loaned, in 1910, 

 three silver disk instruments which were constructed at the cost of 

 the Hodgkins fund and carefully standardized. These instruments 

 were sent to Messrs. M. A. Rykatchew of St. Petersburg, J. Violle 

 of Paris, and C. Chistoni of Italy. Unfortunately the first two 

 mentioned were broken in transportation, but they have since been 

 replaced. In the latest sendings the pyrheliometer proper is wrapped 

 at its heavy end in cotton, and tied into its box in such a way that 

 the projecting thermometer is wholly free, and could not possibly 

 come in contact with any parts of the box. This box is enclosed in 



'See Abbot and Aldrich : Astrophysical Journal, March, 191 1. 



