Influence of the Absorption of Heat on the Formation of Dew. Ill 



of things. The approach of the earth towards the moon will to 

 a certain extent make up for the direct loss of motion due to the 

 influence of the solar wave. But still the moon, which has sus- 

 tained no loss of vis viva, will be moving around the common 

 centre in a shorter period than the earth. This will therefore 

 cause the common centre of gravity to have an exceedingly slow 

 motion around the earth's centre of gravity. This effect may be 

 illustrated experimentally in the following manner : — Let two 

 balls, representing the earth and moon, connected by a rod, be 

 made to revolve upon their common centre of gravity. Retard 

 the motion of the ball representing the earth, and instantly the 

 common centre of gravity (if at liberty to move) will begin to 

 move slowly around the centre of gravity of the ball whose motion 

 has been retarded. 



It is evident, as stated on a former occasion, that the influence 

 of the tidal wave will not only stop the diurnal motion of the 

 earth, but will ultimately bring the moon to the earth's surface. 



XIV. On the Influence of the Absorption of Heat on the Forma- 

 tion of Dew. By Professor Magnus*. 



SINCE heat is emitted by bodies in the same proportion as 

 that in which they absorb it, by the determination of their 

 radiating power their absorption is given, while conversely the 

 absorption gives the radiation. In the case of solid bodies either 

 determination can be readily executed. With liquids the deter- 

 mination of radiation is attended with difficulties ; for as only the 

 surface is free, that is, covered by no solid substance, it is only 

 the radiation in an upward direction which can be observed, in 

 which case ascending currents exert a disturbing influence. A 

 thin layer of liquid adhering to a solid vertical side, as in Leslie's 

 cube, cannot be used for determining radiation ; for it has not 

 everywhere the same thickness, and seldom covers all parts of 

 the solid side ; it is moreover so thin that it transmits the rays 

 of this side itself, so that not merely the action of the liquid 

 alone, but also, with it, that of the solid side is observed. The 

 difficulty of determining the radiation of gaseous bodies is still 

 greater; for they must be used without any solid side. I have, 

 however, made a few determinations of the radiation of dry and 

 of moist air, and some other gases and vapours. Hitherto the 

 capacity of these bodies to transmit heat has alone been deter- 

 mined, and all the heat which did not pass through has been con- 

 sidered to be absorbed, the bodies having been enclosed in glass 

 tubes closed at each end by rock-salt plates. Now there is the 



* Translated from the February Number of the Berliner Monatsberichte. 



