NO. 14 ABBOT SILVER-DISK PYRHELIOMETER — ALDRICH 5 



a journey, screw in the three scretvs as far as they mill go. Their pur- 

 pose is to clamp the silver disk to protect the thermometer during 

 transportation. 



3. Unscrew the two pivots from the sides of the ring of the mount- 

 ing, insert the pyrheliometer so that the thermometer is not next to 

 the worm wheel, and screw in the pivots. 



4. Unclamp the half ring and set the polar axis approximately for 

 the latitude of the place. The thermometer should be next to the upper 

 end of the axis. 



ADJUSTMENTS 



5. For quick adjustment in right ascension guide the pyrheliom- 

 eter with one hand and loosen the lower right-hand milled screw (as 

 seen from the upper end of the polar axis). The worm may then be 

 lowered out of engagement with the wheel and the change made. 



6. To follow the sun, adjust in right ascension and declination until 

 the sun shining through the little hole in the upper plate forms its 

 image on the scratched spot on the nickeled piece below. When ex- 

 posing to solar radiation rotate the worm screw a little (about once 

 every half minute) to follow the sun. 



7. When about to observe, push aside the cover, leaving only the 

 shutter to shade the silver disk. \Mien through with each series of 

 readings, close the cover to keep out dust. 



OBSERVATIONS 



8. When reading the thermometer the observer should hold his head 

 so that the reflection of each dark line of the scale near the degree to 

 be observed, as seen in the mercury thread, is coincident with the 

 corresponding dark line. This prevents parallactic errors of reading. 



9. Having adjusted the instrument to point at the sun and opened 

 the cover, read the thermometer exactly at 20 seconds after the be- 

 ginning of the first minute. Read again after lOO seconds, or at the 

 beginning of the third minute, and immediately after reading open 

 the shutter to expose to the sun. Note that the instrument is then cor- 

 rectly pointed. After 20 seconds read again. After 100 seconds more 

 (during which the pointing is corrected frequently), or at the begin- 

 ning of the fifth minute read again, and immediately close the shutter. 

 After 20 seconds read again. After 100 seconds read again, or at 

 the beginning of the seventh minute. Continue the readings in the 

 above order, as long as desired. Readings should be made within •§• 

 second of the prescribed time. Hold the watch directly opposite the 

 degree to be observed, and close to the thermometer. Read the hun- 

 dredths of degrees first, the degree itself afterward. 



