18 CHANGES OF TEMPERATURE. 



portion of the atmosphere, the spaces between its particles are en- 

 larged and their capacities for containing moisture augmented. 

 Still further we know that "aqueous vapor is highly elastic ; its 

 elasticity, which increases with an increase of temperature, has 

 been determined by Dalton, and its force measured by the height 

 of the mercurial column it is capable of supporting. Aqueous va- 

 por, raised to 32° Fahrenheit, exerts a pressure on the mercury equal 

 to 0.2 of an inch, at 80° to 1.03 of an inch, at 180° to 15.0 inches, 

 and at 212° to 30.0 inches, — a pressure equal to the pressure of the 

 whole atmosphere at the level of the sea." The amount of vapor 

 existing at any time in the air is determined by an instrument called 

 the hygrometer. By means of this instrument we obtain the dew- 

 point. When the readings of both thermometer and the hygrometer 

 are alike, the temperature of the dew-point is the same as that of the 

 air ; the air is then saturated or full of moisture. Quoting further 

 from our generally accepted theory, we say that " it is chiefly in 

 the nights and early mornings of the winter months, that the atmos- 

 phere is saturated with vapor, or that vapor is at its maximum of 

 elasticity for the temperature. In our climate, vapor never attains 

 its greatest elasticity at a high temperature j for if in the summer 

 months the atmosphere becomes saturated it is caused by a decli- 

 nation of the heat, which, contracting the spaces between the par- 

 ticles of the air, squeezes the vapor 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 at- 

 mosphere that Dr. James Hutton founded his theory of rain. He 

 considered rain to be formed by the mixture of two strata of the at- 

 mosphere of different temperatures, and each stratum saturated with 

 moisture. The mean quantity of the vapor contained by the two 

 strata before the mixture being more than the mean heat of the two 

 (after the combination), the excess is precipitated." The princi- 

 pal causes of variance in the weight of the atmosphere are moisture 

 and heat ; the variation is greater in polar than in tropical countries, 

 and in mountainous than in more level regions. We can now un- 

 derstand the importance of the use of the barometer in determin- 



