NO. 4 THE BRIGHTNESS OF THE SKY — MOORE AND ABBOT 3 



meter, and from the ratio of the. nearly equal deflections (first 

 swings) and the known current and the constant of the instrument 

 the calories per square centimeter per minute striking the strips as 

 radiation, can be readily computed by the following formula : 



d. 

 Calories = /v "^c^ 



where ii = constant of the instrument, d,- and Jc = deflections of 

 galvanometer produced by the radiation and current respectively, 

 and c = current corrected by the calibration curve of the milli- 

 ammeter. 



For work in day time, a hollow hemispherical screen of ultra- 

 violet crown glass is placed over the strips to prevent the exchange 

 of long wave length radiation between the strips and the sky. That 

 this admits practically all solar wave lengths of any importance will 

 be shown later in the paper. The transmission of this screen is 

 about 0.92 but the instrument is calibrated with it over the strips 

 so the value of K presupposes its use. 



For day work the instrument can be used in four ways, when it is 

 in a horizontal position: (i) with no shade of any kind, to measure 

 total sky radiation, or if the sun be present, total sun and sky radia- 

 tion ; (2) a small circular polished metal screen can be interposed to 

 shade the strips from the sun but allowing the sky radiation to 

 strike the strips; (3) zones of metal, shading various sections of 

 the sky can also be used ; (4) an enclosing hood can be slipped on 

 which admits only solar radiation when its small shutter is opened. 

 This hood and also the shade mentioned in (2) may be adjusted to 

 follow the sun. 



To determine the value of K, the instrument is used carefully 

 leveled with the hood mentioned in (4) upon it. A series of observa- 

 tions is then taken on the sun, simultaneously with readings of a 

 pyrheliometer, or preferably a pair of pyrheliometers. A set of 

 observations of the altitude of the sun, or of time, are kept, in order 

 to construct an air-mass-time curve. " Air mass " is approximately 

 the secant of the zenith distance, but if taken by the altitude method, 

 the refraction correction must be included. Since the pyranometer 

 strips lie in a horizontal position, while' the pyrheliometer points 

 directly at the sun, the air mass is a factor in the formula for obtain- 

 ing the constant K, which is as follows : 



_ pyrheliometer calories 

 "" c~ idc X dy X air mass 



