14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



from the temperature of the disk read on a thermometer, compute 

 the radiation. He made his observations at Zurich during some 

 clear nights in June and found a nocturnal radiation amounting to 

 0.13 cal. By this method, as well as by the similar method used by 

 Pernter, certain corrections must be made for conduction and con- 

 vection, and certain hypotheses must be made in order to compute 

 the radiation to the whole sky from the radiation to a limited part of 

 it given by the instrument. 



The observations of Pernter a were made simultaneously on the 

 top of Sonnblick (3,095 m.) and at Rauris (900 m.). He observed 

 with an actinometer of the Violle type and found a radiation of 0.201 

 cal. (unless otherwise stated the radiation is always given as 



5-—^— in this paper) at the higher station and o.isi at the lower 



cm. 2 mm. r v s & j 



one. 



Generally the methods for determining the effective radiation out 

 to space have proceeded parallel — with a certain phase difference — 

 with the development of .the methods of pyrheliometry. In the year 

 1897, Homen 2 published an important paper bearing the title " Der 

 tagliche Warmeumsatz im Boden und die Warmestrahlung zwischen 

 Himmel und Erde." His method was an application of a method 

 employed by K. Angstrom for measuring sun radiation. The prin- 

 cipal part of the instrument consists of two exactly equal copper 

 plates. In the plates are introduced the junctions of a thermocouple. 

 If now one of the plates is exposed to the radiation and the other 

 covered, there will be a temperature difference between the disks 

 growing with the time. If at a certain temperature difference, 8, 

 the conditions are interchanged between the disks, they after a 

 certain time, t, will get the same temperature. Then the intensity 

 of the radiation is given by the simple formula : 



Q _2Wh 



"~ t 

 where W is the heat-capacity of the disks. By this method the 

 effects of conduction and convection are eliminated. The weak 

 point of the instrument, if applied to measurements of the nocturnal 

 radiation, lies in the employment of a screen, which must itself 

 radiate and cool, giving rise to a difference in the conditions of the 

 two disks. Homen draws from his observations on the radiation 

 between earth and sky the following conclusions : 



1 Sitzber. der Ak. der Wissensch. zu Wien, 1888, p. 1562. 

 a Homen, Der tagliche Warmeumsatz, etc., Leipzig, 1897. 



