6 



the quantity of vapour then actually existing in it, as 

 will be shown by the following example : 



Let free thermometer ... =63 63 = temp, by free ther. 



Wet thermometer =54 16-2 



Difference = 9 46-8 = dew-point : 



Factor to multiply dif- "I _ , ~ = 



ference By table 0-337 of an inch =- 



16-2 = num- elasticity of vapour in 

 her of degrees to be subtracted from atmosphere, 

 free therm. 



If the readings of the two thermometers be alike, the 

 temperature of the dew-point will be the same as the 

 ^temperature of the air ; and the air will then be saturated 

 With moisture. 



It is chiefly in the nights, and early in the mornings 

 of the winter months, that the atmosphere is saturated 

 with vapour, or that vapour is at its maximum of elas- 

 ticity for the temperature. In our climate, vapour never 

 attains its greatest elasticity at a high temperature; 

 for if in the summer months the atmosphere becomes 

 saturated, it is caused by a declension of the heat, which, 

 contracting the spaces between the particles of the air, 

 squeezes the vapour contained in them closer, and thus 

 brings its elasticity to a maximum for the temperature to 

 which the air has fallen. It was upon the changes of 

 temperature in the atmosphere that Dr. James Hutton 

 founded his theory of rain. He considered rain to be 

 formed by the mixture of two strata of the atmosphere 

 of different temperatures, and each saturated with 

 moisture. The mean quantity of the vapour contained 

 by the two strata before the mixture being more than 



