NO. 19 THE SILVER DISK PYRHEUOMETER — ABBOT 5 



Adjustments 



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

 meter 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. V/hen 

 exposing 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. When 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 

 beginning of the first minute. Read again after 100 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 correctly pointed. After 20 seconds read again. After 100 

 seconds more (during which the pointing is corrected frequently), 

 or at the beginning 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, and imme- 

 diately open the shutter. Continue the readings in the above order, 

 as long as desired. Readings should be made within 1/5 second of 

 the prescribed time. Hold the watch directly opposite the degree 

 to be observed, and close to the thermometer. Read the hundredths 

 of degrees first, the degree itself afterward. 



