RADIATION AND TEMPERATURE. 253 



pass through unchanged, but let B be reduced by one-half in passing 

 vertically through the atmosphere. Then with the sun in the zenith we 



T> 



observe A -t- If we observe the radiation when the sun is 60 from 



a 



p 



the zenith we obtain A + Pouillet's rule would give the outside 



radiation as 



. B 



B 



which is necessarily less than A + B. 



Again, if any ray were totally absorbed by the atmosphere even with 

 vertical sun, Pouillet's method would take no account of it. 



Violle's Actinometer. Violle used an instrument which lessens the 

 fault of Pouillet's Pyrheliometer 

 by diminishing the lag behind the 

 true temperature. A thermometer 

 .t, Fig. 145, is placed in the middle 

 of a double-walled enclosure AB, 

 the space CO between the walls 

 being filled with melting ice or with 

 water at a constant temperature. 



An opening provided with a shutter V \ / / corslant 



,i , f n ,1 \ X. A/ / temperature 



allows the sun's rays to fall on the \ ^ ---- ~ / b ai fi 



thermometer bulb. The shutter 



being closed the thermometer comes 



to the temperature of the enclosure s FIG. 145. Violle's Actinometer. 



which is noted. The shutter is then 



opened and in about fifteen minutes a stationary temperature is reached 



when the heat from the sun balances that lost to the enclosure. To 



find the latter, the shutter is closed and the initial rate of cooling of the 



thermometer is noted. The water equivalent of the thermometer being 



known, we have the heat gained from the sun. Violle found for A the 



value '04233 calories per second per sq. cm. or 2'5398 calories per minute.* 



Langley's Researches with the Bolometer. Langley was the first to take 

 selective absorption by the atmosphere thoroughly into account. In his 

 researches on Mount Whitney f he sought to measure the absorption of 

 each part of the spectrum separately, using a diffraction grating to 

 separate out the various wave-lengths, and a bolometer to compare the 

 heating effect with different thicknesses of atmosphere passed through. 

 His results for wave-lengths between '4/i and I 'Op are given in Fig. 146, 

 ju, being one-millionth of a metre. 



Curve I. is for a given position of the sun at noon, termed " high 

 sun," II. for a position "low sun" in which twice the thickness of 

 atmosphere is passed through. Curve III. is obtained by drawing 

 ordinates bearing the same ratio to those of I. that the ordinates of I. 



* Violle's researches are described in various papers in the Comptcs Rcndus. 

 t Researches on Solar Heat (Washington 1884) : Phil. Mag., xv., 1883, p. 153. 



