4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



in a very hazy sky indicated that the readings were as much as 2-^ 

 percent too high. To diminish the error from this source all silver- 

 disk pyrheliometers made after 1926 carried tubes 32 cm. long in 

 place of the original 15 cm. The exposed sky area was thus reduced 

 from .0043 hemisphere to .0013. As opportunity has arisen the fol- 

 lowing older instruments have been similarly modernized by substi- 

 tuting the longer tube and enlarging the base: S.I. Nos. i, 5, 16, 17, 

 26, 31, 41, 42, and 44. In hazy skies the error due to sky radiation 

 now seldom exceeds ^ percent. 



While it is possible, from a study of the dimensions and physical 

 properties of the instrument, to reduce its readings to heat units, 

 there are uncertainties in thus using it as an absolute standard. We 

 have preferred to consider it as a secondary instrument, the constant 

 of each individual pyrheliometer being determined by careful com- 

 parisons with Smithsonian standard instruments.^ Readings of the 

 silver-disk instrument are proportional to the intensity of radiation 

 of the sun, and are comparable one with another at all times and 

 places. Multiplying the corrected readings by the constant of the in- 

 strument reduces them to true heat units. 



DIRECTIONS FOR USE 



These directions, with minor alterations to conform with our pres- 

 ent practice, are taken from Dr. Abbot's paper, mentioned above. 



SETTING UP 



1. If the mercury column of the thermometer is broken, remove the 

 little screw at the side near the upper end of the nickel-plated tube, 

 and take off the upper portion of the tube. Then heat the exposed 

 stem cavity of the thermometer gently in a smoky flame (a match 

 flame is good) until the mercury is expelled from the cavity. Then, 

 holding the thermometer vertical, shake the instrument repeatedly 

 with a downward jerk until the mercury columns join. 



2. Remove the two little ivory or fiber plugs (using pliers if nec- 

 essary) and unscrew the two brass screws under them, and also un- 

 screw the third similar screw seen through the trunnion on the other 

 side of the case. About three complete turns of each screw is proper. 

 Insert the two ivory plugs. When packing the instrument again for 



3 Descriptions of Smithsonian absolute standard pyrheliometers are given in 

 Annals of the Astrophysical Observatory of the Smithsonian Institution, vol. 

 3, p. 52; vol. 6, p. 5; Smithsonian Alisc. Coll., vol. 87, No. 15; vol. 92, No. 

 13; vol. no, No. 5. 



