76 • METEOROLOGY. 



Now though the an- is very seldom saturated, even over large bodies 

 of water, it must be plain that slight changes in its temperature will 

 produce apparently great changes in its comparative dryness or moisture. 

 It is also in consequence of this law, that the invisible is so readily con- 

 densed into visible vapor or cloud, and thence precipitated in the form 

 of rain and snow, and that these again, on the contrary, are so readily 

 converted into vapor and elevated into the atmosphere, thus producing 

 that endless round of changes the investigation of whose laws consti- 

 tutes the science of Meteorology, and which not only afford a pleasing 

 and useful variety in the world without us, but are also absolutely ne- 

 cessary to enable the earth to sustain vegetable and animal life. It is 

 further a consequence o[ this law, that as the temperature is greatest 

 near the surface of the earth, and thence rapidly diminishes as we ascend, 

 the great mass of vapor must be confined within a moderate distance 

 of the earth. Cloud is seldom found beyond an elevation of 2| or 3 

 miles high. The region of the clouds is that elevation at which the im- 

 perfectly saturated air, when elevated from below, would become cold 

 enough to condense its invisible into visible vapor. This must vary 

 exceedingly at different times. When the air is near saturation the cloud 

 is low, and sometimes rests upon the earth as in mists and fogs ; when 

 the air is a little drier, the cloud is higher; when it is very dry, there is 

 either none at all, or it is to be seen at a great elevation like threads 

 stretched across the sky. 



6. The space moreover in which the cloud exists, contains not any 

 more moisture than that immediately below it, but generally much less, 

 though the former be apparently filled, and the latter seem to contain 

 none. The higher temperature of the space below holds its moisture in 

 an invisible state, whicli that in which the cloud exists cannot do. The 

 steam of boiling water issuing from the spout of a tea-kettle may again 

 be adduced as illustrative of this point. 



This lower space, in which ordinary cloud does not form, because it 

 is not saturated with moisture, is nevertheless almost constantly ob- 

 scured by vapor in an incipient state of condensation, ordinarily called 

 smoke. The difference between this and ordinary cloud is, that the latter 

 is formed by rapid condensation in a space nearly saturated, and there- 

 fore consists of larger particles, and the former by slow condensation 

 in a space in which the moisture is increasing, but yet below saturation. 

 Its particles are small and not sufficiently near each other to produce a 

 dense cloud. 



These preliminary statements will greatly facilitate our discussion 

 of points presently to be taken up. 



