172 HOT WATER AND STEAM. 



The constitution of the atmosphere has a con- 

 siderable influence on the growth of plants, and 

 more particularly its relation to moisture. The 

 variable state of the moisture in the air, was not 

 registered with any degree of accuracy, until 

 within these few years, which improvement may 

 be attributed chiefly to instruments which have 

 been invented for the purpose ; but^ even now, 

 none of them are so perfect as it would be de- 

 sirable to render them. 



When air is cooled below a certain degree, it 

 deposits a portion of the moisture it contains, in 

 the form of dew, and when air deposits dew, it 

 is saturated with moisture, therefore, air always 

 contains that quantity of moisture which would 

 saturate the same quantity of air when its tem- 

 perature is reduced down to the point at which 

 it deposits dew. 



The ordinary state of the atmosphere, as 

 respects moisture in this country, is extremely 

 variable ; but the mean result of many obser- 

 vations of the thermometer, and also the dew 

 point, shows that the temperature of deposition 

 and the actual temperature follow, each other in 



