portion of vapour, which exerts an influence, varying 

 according to circumstances, upon the mercurial column ; 

 it is derived from the spontaneous evaporation of water 

 from the surface of the earth, and is called aqueous 

 vapour. Evaporation is promoted by dry air, by wind, 

 by a diminished pressure, and by heat; the quantity 

 evaporated is dependent upon temperature; for heat 

 expanding the gaseous portion of the atmosphere, the 

 spaces between its particles are enlarged and their ca- 

 pacities for containing moisture augmented. Aqueous 

 vapour is highly elastic; its elasticity, w.hich increases 

 with an increase of temperature, has been determined by 

 Dalton, and its force measured by the height of the mer- 

 curial column it is capable of supporting. Aqueous va- 

 pour, raised at 32 of Fahrenheit, exerts a pressure on 

 the mercury equal to O2 of an inch, at 80 to 1'03 

 inch, at 180 to 15-0 inches, and at 212 to 30-0 inches, 

 a pressure equal to the pressure of the whole atmo- 

 sphere at the level of the sea. The quantity of vapour 

 existing in the atmosphere is measured by an Hygrometer. 

 The one now in general use consists of two thermo- 

 meters, one bulb of which being covered with muslin 

 and kept constantly moist, will, according to the quan- 

 tity of evaporation at the time of observation, stand 

 lower than the other bulb, which being left free gives 

 the temperature of the air : from the difference of the 

 readings of the two thermometers, we are able by a very 

 simple rule to obtain the dew-point, or that degree of 

 the thermometer to which the temperature of the air 

 must fall for the atmosphere to become saturated with 



