302 Analyses of Books. [Oct. 



from other bodies. Dew is not deposited indiscriminately upon all 

 bodies. More is deposited on grass than upon gravel or soil. 

 Polished metals have but little deposited on them, so that they are 

 of- en found drv when other bodies in their neighbourhood are wet 

 with dew. Between metals themselves there is a difference in this 

 respect : some heing more readily wetted with dew than others. 

 Platinum is one of the easiest. Iron, steel, zinc, and lead, are more 

 easily wetted with dew than gold, silver, copper, and tin. Dew is 

 deposited in towns as well as in the country, but not so copiously, 

 owing to a variety of causes whicli may be easily conceived. Thus, 

 Dr. Wells found dew deposited in London, but in much smaller 

 quantity than in a garden in Surrey, about a mile and a quarter 

 from town. 



If we examine the temperature of those substances on which dew 

 is deposited, we shall find them colder than the air in their neigh- 

 bourhood. Thus, grass wet with dew was found J°, S°, 9°, 10°, 

 11°, and even Vl° colder than the air four feet above it. This 

 difference of temperature begins soon after the heat of the day has 

 declined : but this difference only holds in clear and serene wea- 

 ther. In cloudy and windy nights the grass was not colder than the 

 air: sometimes it was even warmer; but on such nights no dew 

 was deposited. If the sky become cloudy during the night, the 

 temperature of the grass rises. Thus, on such a night, the grass 

 which was at first 12° colder than the air became only 2° colder. 

 During a fog this increase of temperature sometimes takes place, 

 sometimes not; showing clearly that the change does not depend 

 upon the fog, but the -■'otidiness of the sky. 



The quantity of dew deposited upon bodies is proportional to their 

 Coldness, when compared with that of those in their neighbour- 

 hood. Thus, wool on the upper surface of a board was 9° colder 

 than wool upon the under surface of the same board, and it con- 

 densed more titan three times as much dew. Much more dew is 

 deposited upon grass than upon a gravel walk. On one night in 

 which this difference was very striking, the grass was 16i° colder 

 than the gravel wrdk. The same observation applies to garden 

 mould, which docs not condense so much dew, nor become nearly 

 so coid as grass. 



On dewy nights the temperature of the earth an inch or an inch 

 and a half under the surface is much warmer than that of the grass. 

 In one case the observed difference was from 12° to 16°. 



The heat of metals is not correctly ascertained by placing the 

 naked bulb of a thermometer on them. Unless the bulb be co- 

 vered by a coating of gilt paper, or something similar, it indicates 

 a much gre; .. r degree of cold than the metal really has. 



Bright metallic plates on dewy nights are much warmer than the 

 grass on which they are placed, and usually as warm as the air four 

 feet above them : but when dew forms on them they are always 

 colder than the air. But metals do not become so cold as other 

 bodies by exposure : they were never observed more than 3° or 4° 



