250 WRITINGS OF JOSEPH HENRY. [1855- 



apparent size of the moon was readily perceptible.* When 

 this instrument was directed first to the clear sky in the 

 vicinity of a cloud, and then immediately after to the cloud 

 itself, the needle of the galvanometer attached to the thermo- 

 electric pile in the tube always deviated several degrees. At 

 first sight it might appear from this experiment that the heat 

 of the cloud was greater than that of the transparent air in 

 which it was floating, but this was not necessarily the case; 

 the rays of heat from the apparatus when it was directed into 

 the clear sky passed off into celestial space, while when the 

 instrument was directed to the cloud they were absorbed and 

 radiated back. It is probable however that the lower surface 

 of the cloud is really a little warmer than the air in which 

 it is floating, from the radiation of heat by the earth, while 

 the upper surface is probably colder on account of the un- 

 compensated radiation into space. But be this as it may, the 

 counter radiation of the clouds prevents the sufficient cool- 

 ing down of the bodies at the surface of the earth for the 

 deposition of dew, or at least for the formation of a copious 

 quantity. A haziness of the atmosphere (and it is prob- 

 able a large amount of invisible vapor) will retard the 

 radiation, and hence a still, cloudless night, without a depo- 

 sition of dew, is considered as indicative of rain. The 

 amount of deposition of dew will also depend upon the still- 

 ness of the atmosphere ; for if a brisk wind be blowing, the 

 different strata of air will be mingled together, and that 

 which rests upon the surface of the ground will be so quickly 

 displaced as not to have time to cool down sufficiently to 

 produce the deposition. 



Again the deposition will be more copious on bodies the 

 surfoces of which are most cooled by the radiation. It is 

 well known that different substances have diff'erent radiating 

 powers. The following table from Becquerel exhibits the 

 proportional tendency of diff'erent substances to promote the 

 deposition of dew. The figures do not represent the relative 

 emisssive power, but the combined effects of emission and 

 conduction : 



^[See ante, vol. i, p. 283.] 



