2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



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 transportation. These screws must be loosened dur- 

 ing observations. 



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

 instrument from temperature changes. This box is also halved to- 

 gether and fastened by long wood-screws, one of which is seen near 

 the letter, d, in the figure. 



Sunlight may be admitted through the tube, c. This tube is pro- 

 vided with a number of diaphragms, f^ fo is, having circular aper- 

 tures. The aperture, fg, 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 alco- 

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

 secure a fine, even coat, the mixture is filtered through cheese- 

 cloth 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 provided 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 supports the thermometer tube, and the 

 axis of the exposing shutter just mentioned. 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 towards the sun. 



The pyrheliometer is carried upon an equatorial stand, as shown 

 in the figure. A worm and wheel mechanism is provided for fol- 

 lowing the sun. No clockwork is needed, as it is sufficient for the 

 observer to move the worm slightly two or three times a minute. 



The purpose of the silver disk pyrheliometer is merely to furnish 

 readings proportional to the intensity of radiation of the sun, and 

 comparable one with another at all times and places, but not to fur- 

 nish independent means of reducing these readings to true heat 



