NO, 14 ABBOT SILVER-DISK PYRHELIQMETER ALDRICH 3 



during transportation. The thermometer is graduated to tenths of degrees 

 centigrade from —15° C. to +50° C. Two points, 0° and +50°, are first 

 marked on the stem by the makers, and then the thermometer is graduated 

 by equal linear intervals without regard to the variations of cross-section of 

 bore of the stem. Before insertion in the instrument, a careful calibration 

 of the thermometer stem is made. 



The silver disk, a, is enclosed by a copper cylindrical box, c, halved together 

 for convenience in construction. Three small steel wires, not shown in the 

 figure, support the silver disk. These wires lie in the plane of the center of 

 the disk at 120° intervals apart. Midway between them are three brass screws, 

 not shown, which may be screwed through the walls of the box, c, up to their 

 heads. These screws in that position clamp the silver disk tightly. Their 

 purpose is to prevent the breakage of the thermometer if jarred during trans- 

 portation. These screws must be loosened during observations. 



The copper box, c, is enclosed by a wooden box, d, to protect the instru- 

 ment from temperature changes. This box is also halved together and fas- 

 tened by long wood-screws, one of which is seen near the letter, d, in the figure. 



Sunlight may be admitted through the tube, e. This tube is provided with 

 a number of diaphragms, /i, ji, /a, having circular apertures. The aperture, jz, 

 nearest the silver disk is slightly smaller than the others, and slightly smaller 

 than the disk itself. Thus it limits the cross-section of the sunbeam whose 

 intensity is to be measured. The entire interior of the tube, e, the box, c, 

 and the silver disk, a, are painted dead black with lamp-black mixed in alcohol, 

 with a little shellac added to cause the lamp-black to stick. To secure a fine, 

 even coat, the mixture is filtered through cheesecloth before applying, and on 

 the disk it is warmed with an alcohol lamp until the brush marks disappear. 



A rotatable shutter, g, with three nickeled parallel metal plates, h h h, is pro- 

 vided for cutting off the sunlight as desired. The top of the tube, e, carries 

 a screen, k, large enough to shade the wooden box, d. This screen also sup- 

 ports the thermometer tube, and the axis of the exposing shutter just men- 

 tioned. A small hole in the part which supports the thermometer admits a 

 little guiding beam of sunlight, i, whose use is to assure the observer that the 

 instrument points toward the sun. 



The pyrheliometer is carried upon an equatorial stand, as shown in the figure. 

 A worm and wheel mechanism is provided for following the sun. No clock- 

 work is needed, as it is sufficient for the observer to move the worm slightly 

 two or three times a minute. 



In 1927 two alterations were adopted: (i) the tube e was length- 

 ened and (2) the base was enlarged to counteract top-heaviness due 

 to the longer tube. These changes are shown in the photograph, plate 

 I. The short tube form of 1909 when the shutter was opened exposed 

 each point on the silver disk to a cone of sky io°38' in diameter. Since 

 the sun subtends a diameter less than i/20th as great, the sky area 

 exposed is at least 400 times that occupied by the sun. In very clear 

 skies the radiation from this area of sky is negligible as compared to 

 that from the sun. But measurement at Mount Harqua Hala in 1925 ^ 



2 Annals of the Astrophysical Observatory of the Smithsonian Institution, 

 vol. 5, p. 83. 



