KEPOET OF THE SECEETAKY. 83 



matters. The preparation for the eclipse work occupied some time 

 of the director and of the instrument maker. 



Several investigations relating to the war, a brief note of which 

 was mentioned in last year's report, were continued during the fiscal 

 year. 



The painstaking and valuable work which Mr. Fowle has been 

 doing on the revision of the Smithsonian physical tables should re- 

 ceive some notice, although this work is being done by him outside 

 of his regular hours of service for the observatory. This book has 

 passed through a number of editions under his editorship and has 

 attained an enviable reputation in this country and abroad for the 

 accuracy and fullness of its contents. The new edition, which is now 

 in press, has received unusual attention on his part, and very valu- 

 able cooperation from the various scientific departments of the 

 Government and of outside individuals in colleges and industrial 

 corporations and elsewhere, and will be a great advance over any 

 of the former editions. 



In connection with work of the Observatory, we have long wished 

 to determine the solar constant of radiation by a method which 

 does not involve the assumption that the transparency of the atmos- 

 phere is constant over the two or three hours required for the deter- 

 mination of it by the usual spectrobolometric method. We hoped 

 that, seeing that the sky is brighter when the transparency is less, 

 an observation by the pyranometer, or some other more suitable 

 instrument, of the brightness of the sky in the neighborhood of the 

 sun, combined with the usual measurements of the pyrheliometer 

 and perhaps of the spectrobolometer, but only at one period of time, 

 might be sufficient to determine the solar constant by a satisfactory 

 empirical process based upon spectrobolometric investigations of 

 former years. In the hope of getting an instrument more satisfac- 

 tory than the pyranometer for this special work, a new design com- 

 prising essentially two disks, one of which is shined upon through 

 a graduated aperture by the sun and the other of which is exposed 

 to the small region of sky desired and both connected by thermo- 

 electric junction so as to enable equality of temperature of the two 

 disks to be adjusted, was devised and partly constructed at Wash- 

 ington. It was sent in a letter to Calama, Chile, and was finished 

 by the director during his visit in Chile and is now in satisfactory 

 operation, although it has not yet supplanted the pyranometer for 

 the purpose in question. 



Another problem which requires a new kind of apparatus is the 

 measurement of nocturnal radiation such as the earth sends out to 

 space. This investigation is exceptionally difficult, for it involves a 

 range of wave length from 5 microns to 50 microns. There is no 

 surface either of blackened metal or other substance which is fully 



