m 



A COMPARTSO^ BETWEEN THE 



as not to affect, in any way, the senses or functions of the human 

 frame. Whether these noxicus exhalations always descend when 

 the aqueous vapour which bears them upward is condensed, or whe- 

 ther, as in the just-mentioned theory of aerolites, they remain in 

 the upper regions some indefinite period, and gradually, by some 

 natural operation in the higher regions, accumulate, become con- 

 densed, and then again descend, (not to beat our brains out, as the 

 aerolite undoubtedly would, were we to encounter it, but sometimes 

 to fall with a hardly less fearful, though invisible, blow), is a ques- 

 tion not easy, perhaps, to determine. However difficult it may be 

 to admit this theor}-- in the instance of meteoric stones, it is far from 

 being so with respect to malarious exhalations. The experience we 

 have of them, in the production of various epidemical diseases inci- 

 dent to man, renders it highly probable that they do very often 

 descend in a noxious form, when the aqueous vapour with which 

 they are associated is condensed ; and we can believe that they may 

 sometimes remain and accumulate until some unknown operation in 

 the higher regions favours their descent — not, indeed, in a solid and 

 visible state, but in a sufficiently concentrated and invisibly cloud, 

 like form as to sweep successively over various parts of the earth's 

 surface, occasioning the appearance of violent epidemic diseases. 



We by no means intend to be understood as implying that the 

 evaporation taking place from every part of the surface of our globe 

 necessarily disentangles noxious exhalations from their innocent re- 

 pose; but, only, that where the surface does contain any materials 

 for their formatiott, there the aqueous vapour promotes their disen- 

 gagement from the ground — is the agent through whose means 

 they escape — and associated with which they rise into the higher 

 regions. It must, therefore, be more especially important to con- 

 sider as closely as possible, the various forms and changes to which 

 this, the more tangible material, (if we may use the expression), is 

 subject. 



It is with these views that the Terrestrial Radiation of 

 Caloric, — ^a process which, with a still and clear atmosphere, speed- 

 ily reduces, at night, the temperature of various parts of the earth's 

 surface and the superincumbent air, — becomes a highly important 

 object for considei'ation. Radiation is much modified by situation. 



