NO. 14 ABBOT SILVER-DISK PYRHELIOMETER ALDRICII 7 



16. (Note.) The approximate method of procedure stated in (13) 

 and (15) is much easier than the exact method, and having been 

 found by experiments to yield closely comparable results under all 

 circumstances of use, within the error of measurement, it has been 

 adopted, and the standardization of the instrument is made by this 

 method. 



17. To reduce the result R^i to standard calories per square centi- 

 meter per minute, or to the Smithsonian scale of 1913, multiply by 

 the factors furnished from the Smithsonian Institution with each in- 

 strument. 



18. Example of reduction : 



Number i 2 3 4 5 6 



Reading I5?I2 I4?25 i4?8o I7?s8 I7?36 i6?09 



Differences 0^87 2?78o i?27 



Cooling correction.. i?070 



Ri .... 3-850 



Tx .... i6?2 



Scale correction . . . — 0.0020Ri= — o?oo8 



KR(T-30°) -(o.ooii)(i3.8)Rt= -o?058 



Air correction +o.ooo7Ri= +0?003 



RS .... 3?787 



PRECAUTIONS 



Constant watchfulness and care are needed to keep the instrument 

 in best condition. For example, it should not be inverted or jarred, 

 thus keeping the mercury column and the wax bond between 

 thermometer and silver disk intact. The blackening of the silver disk 

 should be examined periodically. If there is evidence of specks or 

 spottedness, the surface should be carefully brushed with a soft 

 camel's hair brush fastened to a long slender handle. The cover at 

 the end of the tube should be kept closed when not in use. With 

 care the blackness remains unchanged over many years. Our two 

 substandard silver-disk instruments, A. P.O. No. Stis and S.I. No. 5 

 are evidence of this. Repeated comparisons against the absolute water- 

 flow pyrheliometer over a period of 20 years have shown no evidence 

 of change. 



A silver-disk pyrheliometer ready for shipment, including stand- 

 ardization and boxing, costs the Smithsonian Institution about $200. 

 While it is not desired to manufacture them extensively, it has been 

 our practice to prepare and sell these pyrheliometers to individuals 

 and institutions likely to use them for valuable and regular solar ob- 

 servations. The special reading glass above mentioned can be fur- 

 nished for $10 additional. 



