74 REPOET OF THE ACTING SECRETAKY. 



Volume II of Anuals of the Astrophysical Observatory. Considerable time has 

 been occupied by the aid acting in charge in the preparation of the text for 

 this volume, which it is hoped to publish during the coming fiscal year. 



Computation of results. — The reduction of observations of solar radiation and 

 solar absorption has occupied the main portion of the time of the junior assist- 

 ant and computer, but without interfering with the continuation in Washington 

 of the series of observations on the absorption of the solar envelope and the 

 solar radiation. Improved methods of reduction have been devised, so that the 

 task of reducing a single day's work is not as heavy as formerly. 



Improvements of apparatus. — In earlier reiwrts the principles embodied in a 

 new form of standard pyrheliometer have been set forth. It may be recalled, 

 however, that in this instrument the solar radiation, which passes at right 

 angles through a circular aperture of known area, is principally absorbed at 

 the rear end of a cylindrical hollow chamber, and that such portions of heat or 

 radiation as escape from the rear end of the chamber are absorbed at other 

 points along its walls, so that heat is almost wholly prevented from escaping at 

 the entrance. The heat thus fully absorbed within the chamber is taken up by 

 a current of water which flows steadily at a measured rate in a spiral course 

 round the walls of the chamber, entering at the front and leaving at the rear ; 

 and the rise of temperature of the water is determined by a platinum resist- 

 ance thermometer. Thus the rate of solar radiation is measured by the 

 rise of temperature it produces in a known amount of water. A definite 

 check on the accuracy of the instrument is had by introducing a measured 

 electric current through a coil of wire situated within the chamber, thus pro- 

 ducing a known amount of heat there. The agreement between this amount of 

 heat and the amount carried ofC by the flowing water, as determined by means 

 of the recording apparatus, is the evidence of the excellence of the pyrheliometer. 

 In practice the instrument has now come to a high state of perfection, and 

 forms a valuable part of the equipment on Mount Wilson. Various improve- 

 ments of the means of promoting a satisfactory circulation of the water and of 

 measuring its rise of temperature have been introduced during the year. 



Several copies of an improved form of secondary pyrheliometer have been 

 made at the Observatory shop and added to the equipment. This instrument, 

 though coming by. a kind of evolution from Pouillet's water pyrheliometer, 

 has now become very difi:erent from it. The receiving surface is a copper 

 disk blackened l)y smoke. A cylindrical bulb thermometer is inserted radially 

 in a hole at the side of the disk, and its connection with the disk improved 

 by filling the hole with mercury, which is prevented from spilling by packing 

 at the mouth of the hole. A hollow copper sphere incloses the disk, and this 

 is protected from outside temperatures by being inclosed in a wooden sphere. 

 The solar rays pass down a diaphragmed tube and fall at right angles upon 

 the copper disk. When reading the instrument, it is alternately shaded and 

 exposed to the sun at two-minute intervals, and the rate of rise of temperature 

 of the disk is determined from thermometer readings made each twenty sec- 

 onds. Readings with this instrument must be compared with those of the 

 standard pyrheliometer to reduce them to the absolute scale, but as between 

 themselves the readings appear to be accurate to half of 1 per cent indepen- 

 dent of wind or temperature outside. 



The bolometer also has received very essential improvements, one of them 

 consisting of a device for abolishing "drift" in situations whei'e it is not 

 possible to maintain the bolometer at uniform temperature. This is done by 

 Introducing as a part of one of the balancing coils of the bolometer a little 

 copper wire in place of an equal resistance of wli'e of zero temperature coeffi- 



