NO. 3 RADIATION OF THE ATMOSPHERE ANGSTROM 1 3 



nocturnal cooling of bodies exposed to the sky, a cooling that is 

 evidently not only due to radiation but is also influenced by conduc- 

 tion and convection of heat through the surrounding medium. 

 Melloni, making experiments in a valley called La Lava, situated 

 between Naples and Palermo, found that a blackened thermometer 

 exposed on clear nights showed a considerably lower value (3.6 C.) 

 than an unblackened one under the same conditions. Melloni draws 

 from his experiments the conclusion that this cooling is for the most 

 part due to the radiation of heat to space. In fact, such a cooling 

 of exposed bodies below the temperature of their surroundings was 

 very early observed. Natives of India use it for making ice by ex- 

 posing flat plates of water, on which dry grass and branches are 

 floating, to the night- sky. The formation of ice, due to nocturnal 

 radiation, has been systematically studied by Christiansen. 



So far the observations have been qualitative rather than quantita- 

 tive and the object of the observations not clearly defined. The first 

 attempt to measure the nocturnal radiation was made by Maurer, 

 the Swiss meteorologist. In the year 1886, Maurer published a 

 paper dealing with the cooling and radiation of the atmosphere. 1 

 FrJp thermometrical observations of the atmosphere's cooling he 

 deduces a value 8 = 0.007. io -4 (cm. 3 min.) for the radiation coefficient 

 of the air and from this a value for the radiation of the whole atmos- 



C3.1 



phere: o.^Q tt- '—. — at o°. This value is obtained on the assump- 



r cm.- mm. 



tion that the atmosphere is homogeneous, having a height of 8.10 5 

 cm; and by the employment of the formula 



R=^-[i-e- ah ] 



where S is the radiation, a the absorption coefficient and h = S.io°. 

 Maurer's manner of proceeding in obtaining this value can scarcely 

 be regarded as quite free from objection, and in the theoretical part 

 of this paper I shall recur to that subject. But thr6ugh his theory 

 Maurer was led to consider the problem of the nocturnal radiation 

 and to measure it. 2 His instrument consisted of a circular copper 

 disk, fastened horizontally in a vertical cylinder with double walls, 

 between which was running water to keep the cylinder at a constant 

 temperature. The cover of the cylinder was provided with a cir- 

 cular diaphragm, which could be opened or shut. Opening and 

 shutting this diaphragm at certain intervals of time, Maurer could, 



1 Meteorologische Zeitschrift, 1887, p. 189. 



2 Sitzber. der Ak. der Wissensch. zu Berlin, 1887, p. 925. 



