LECTURES AND ADDRESSES. 



nights it is the latent heat of the vapour condensed 

 into dew. This vapour is taken chiefly from the 

 air engaged among the stems and leaves, which, in 

 the case at least of fine grass, is all nearly at the 

 same temperature as the leaves ; the temperature 

 of the surface of these being of course rigorously 

 the same as that of the air in contact with them. 

 Thus the temperature of the leaves can never go 

 below the (few-point of the air touching them, and 

 any cooling which they experience after dew begins 

 to deposit upon them is only equal to the lowering 

 of the dew-point, occasioned by the amount of 

 drying experienced by the air in consequence of 

 the condensation of vapour out of it. 



Clouds, as remarked first by Prevost, being 

 practically opaque, prevent the surface of the 

 earth from tcnding\>y radiation to a lower tempera- 

 ture than their own, which, unless they arc very 

 high, is generally not much colder than the dew- 

 point of the lower air, but is at all events in 

 ral Mifllcicntly warm to prevent the finest 

 blades of grav- from acquiring any very sensible 

 dew, or to allow the general temperature of grass 



