NO. II STANDARD PYRHErjOMIiTRY — ABBOT 3 



of carrying off absorbed lieat in a flowing stream of water, the 

 chamber was immersed in a water bath whose rate of rise of tempera- 

 ture, and coohng corrections, were observed after the methods of 

 exact calorimetry. In this instrument only one insulated coil of wire 

 was introduced, but it was wound in part within the wall of the sides 

 of the chamber. Thus it had almost identically the same facility t(j 

 give up its heat to the water as did the solar rays. Tests of electrical 

 heating with this instrument were made on October 24 and 26, 1912, 

 and recorded on page (^"j of Annals, volume 3. Six tests gave 100.05 

 percent of heat found, and the results are even more consistent than 

 the excellent ones with the water-flow pyrheliometer. Silver-disk 

 pyrheliometer APO 8|,is, which we have ever since used as secondary 

 standard, was compared on a number of occasions from 1910 to 1912, 

 some at Washington, others at Mount Wilson, and with both the 

 water-flow and the water-stir standards. The results are given at 

 the bottom of page 70, Annals, volume 3. They give the following 

 independent determinations of the constant for APO Sbis : 0.3798, 

 0-3791. 0.3809, 0.3786, 0.3792, 0.3770, 0.3772. 



Many years later the silver-disk pyrheliometers were altered to 

 have longer vestibules so as to reduce the angular area of sky near 

 the sun to which they were exposed. The water-flow standard pyr- 

 heliometer was also changed. A Russian, V. M. Shulgin, made the 

 valuable suggestion that by using two chambers rather than one in the 

 water-flow pyrheliometer, with the water stream divided just at the 

 entrance of their walls, inequalities in rate of water flow would be 

 the same in both. Plence if the solar heating in one chamber was 

 continually being balanced by electrical heating in the other, the 

 inequalities of flow of water would cease to produce fluctuations in 

 the readings. In 1932 we introduced Shulgin's method, and, depending 

 on the results oj iqio to 1912, to the effect that solar heating and elec- 

 trical heating are equally efficiently absorbed, all subsequent standard- 

 izations of pyrheliometers by Smithsonian observers are based on the 

 use of the standard water-flow pyrheliometer as an electrical compen- 

 sation instrument. That is, we no longer measure the water-flow rate, 

 or the rise of temperature of the water, but we balance solar heat in 

 one chamber against electrical heat in the other, and reverse chambers 

 as respects heating again and again. I repeat, ice noxv absolutely 

 depend on the experiments I have quoted, of the years iQio to 10T2, 

 which prove that in our pyrheliometer electrical heat and solar heat 

 are both fully absorbed in the water stream. 



Prior to the adoption of W M. Shulgin's suggestion of using two 

 chambers in the water-flow pyrheliometer, we found great difficulty 



