METLOKOI.UGY. 75 



yet invisible by reason of its liigh temperature ; but bcycnul about an 

 inch from the spout, the vapor is rendered visible by reason of its di- 

 minished temperature, and there it appears like a cloud. Visible vapor 

 exists in two states ; viz. that in which the particles are too small to be 

 separately seen, and that in which they are sufficiently large to be seen 

 either by the naked eye or by a magnifier. The forjner is dry, or does 

 not moisten objects with which it comes in contact, and, though its par- 

 ticles are not separately visible, when we look through its mass, it ob- 

 scures the air and has all the appearance of smoke. Indeed genuine 

 smoke does not perhaps owe its visibility much more to the volatilized 

 carbon than to the moisture it contains. The latter is generally wet, 

 and constitutes what is ordinarily called fog and cloud. Tldrdhj^ mois- 

 ture exists in a state of greater condensation as rain^ kail and snow, &c. 



4. The quantity of invisible vapor in the air. The quantity of invis- 

 ible vapor capable of existing in any portion of the atmosphere, is de- 

 pendent upon its temperature. The idea entertained by most persons, 

 that it is the air which produces the evaporation, is not sustained by ex- 

 periment. At a given temperature the same quantity of moisture will 

 exist in a given space, wliether that space be a vacuum, or contain air. 

 The air actually forms a mechanical impediment to the formation of va- 

 por, so that the space will not be filled as soon if air be present as if it 

 be a vacuum. C\n-reiits do indeed promote evaporation, but it is by 

 continually sweeping away the vapor already formed, and thus giving 

 room for the formation of more. 



The quantity of vapor in the same space at different temperatures, 

 does not bear a direct ratio to the temperature. Within the ordinary 

 range of the temperature of the atmosphere near the earth, the qtianlity 

 of vapor is nearly doubled for every additional increase of twenty de- 

 grees in temperature ; so that when the air becomes warmer, the quanti- 

 ty of vapor capable of existing increases, and when it becomes colder, 

 diminishes more rapidly than the temperature. 



5. It is a consequence of this law, that if a space saturated at 80°, 

 that is, holding as much vapor in an invisible state as possible, were 

 cooled to 60°, it would lose by condensation one half of its moisture; 

 or 6| cubic yards of space would aflbrd one cubic inch of rain. Or, 

 since the atmosphere grows colder as we ascend, by 1° for every 352 

 feel, if 6| cubic yards of air saturated with moisture at 80°, were eleva- 

 ted to the height of 2300 yards, it would become cloud, and deposit 

 one inch of rain. On the conlrary, air saturated with moisture at 60°, if 

 heated to 80°, would tiien be only half saturated, or v>ould become com- 

 paratively dry. 



