on the Optical Phenomena of the Atmosphere. 3 



blackened surface may radiate against an unclouded portion of 

 the sky. After somewhat more than four minutes, the surface 

 is covered with a screen and directed perpendicularly against the 

 sun's rays. At the end of the fifth minute the screen is removed, 

 and the apparatus is permitted to remain five minutes longer in 

 the sun. At the end of the tenth minute it is again, as at first, 

 brought into the shade and permitted to radiate five minutes 

 longer. This operation can be often repeated if necessary. The 

 two quantities which are here to be made use of are, the increase 

 of temperature during the five minutes' exposure to the sun, and 

 the decrease of temperature during the periods of radiation im- 

 mediately before and after; the arithmetical mean of the two 

 latter may be regarded as the quantity of heat lost by radiation 

 during the five minutes in the sun. The action of the sun {t) 

 alone will therefore be 



1+ 2 ' 



where T represents the observed temperature, r and r 1 the amounts 

 of the radiation before and after. 



After the second radiation we generally placed the instrument 

 once more in the sun, and afterwards permitted it again to ra- 

 diate ; in this way two series of observations were obtained. 



To secure perfect accuracy in the experiments, it would be 

 desirable that the temperature of the air during the time occu- 

 pied by each should remain constant ; for any inconstancy in 

 this respect would be accompanied by an increased or diminished 

 ' cooling. But in experiments which occupy fifteen minutes and 

 upwards, trifling alterations are unavoidable; by taking the 

 arithmetic mean, however, they are sufficiently compensated. 



Of the experiments with the pyrheliometer, the maxima alone 

 are chosen for nearer consideration ; for in these cases only is it 

 probable that no turbidity of the atmosphere by vapours, fine 

 veils of clouds, &c. takes place. In making such experiments, 

 deviations in the transparency are often recognised which are 

 totally inappreciable with the telescope or with the naked eyes, 

 but which afterwards announce themselves by the presence of 

 thin clouds, &c. 



In the observations on the Johannishutte (7581 P. F.), the 

 maxima of the increase* in August and September 1848 were 

 4*9 to 5°'2 C. ; the mean height of the barometer at the time of 

 the experiment being =571 millims. All the observations were 



* Under ' increase ' is here to be understood the increase of temperature 

 with the radiated heat ( -^— ) added to it. 



B2 



