10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



gives the same relative readings compared with the one at the 

 Astrophysical Observatory that it did in 1907. 



5. As regards accidental error of observation, persons with good 

 eyesight and experience in observing appear to read with a probable 

 error, for a single determination, at high sun, not exceeding 3/10 

 per cent. As a single determination depends on six readings, and 

 the rise of temperature determined is only about 3°, this requires a 

 probable error of single readings of temperature not exceeding 

 o.°oo5, or 1/20 division of the scale. It seems almost incredible 

 that this degree of accuracy should usually be attained, but compari- 

 sons of instruments by two observers simultaneously, if made under 

 excellent sky conditions, so indicate. 



A silver disk pyrheliometer ready for shipment, including stand- 

 ardization and boxing, costs the Smithsonian Institution about $100. 

 While it is not desired to manufacture them extensively, the Insti- 

 tution has in several instances consented to prepare and sell them. 

 In cases where this evidently will tend to promote valuable and 

 regular series of solar observations, the Institution is prepared to 

 furnish silver disk pyrheliometers at the price above stated. 



