382 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



bottle getting hot, as would be the case if the sides of the vessel 

 were impermeable. 



It is this evaporation, so abundant and so rapid on clear nights, 

 which gives rise to the formation of dew, which is a condensation of 

 the vapour in the air in little drops on the surface of exposed objects. 

 When the resulting cold is sufficiently intense the drops freeze and 

 produce hoar-frost. In Bengal, where the temperature is too high for 

 ice ever to form naturally, it is obtained artificially in the following 



FIG. 263. Graduation pile for the evaporation of salt waters. 



manner. Tyndall explains in these terms l the process employed, and 



the cause of the physical phenomenon of which it is an application : 



" Wells (the author of The Theory of Dew) was the first," he says, 



" to explain the formation artificially of ice in Bengal, where the 



substance is never formed naturally. Shallow pits are dug which are 



partially filled with straw, and on the straw flat pans containing 



1 Heat as a Mode of Motion, p. 461. 



