186 



CHAPTER OF VARIETIES. 



low ground after sunset, is merely the moisture precipitated from the 

 air upon the earth cooled by radiation. — W. E. H. 

 19th Feb. 1833. 



On the objections of mr. hart, page 76, respecting 

 the theory of rain. — The whole of Mr, Hart's objections are 

 highly curious and interesting, and the subject has always much puz- 

 zled me, particularly the fact, that a hot sun will dry a wet pavement 

 by heat, and a cold wind, if strong, though there is no sun, will produce 

 the same effect often in the same time. 



Objection 1. The reason of this seems to be, that the vapour rises 

 imperceptibly, both above and below 60°, and is condensed in the at- 

 mosphere when it is below that temperature. Thus the vapour is im- 

 perceptible in the neck of a Florence flask boiling over a lamp, though 

 clearly seen above it, when the temperature is less. Again, when a 

 draft of cold air dries a wet pavement, no steam is perceptible on the 

 surface, though five or six feet above it may be felt as a sensible 

 dampness on the clothes and linen. 



Objection 2. In this case the temperature is raised both in the air 

 and on the surface of the earth. We know not what becomes of water 

 when raised in the atmosphere, by a high or very low temperature, as 

 in both cases it is imperceptible, though its particles are no doubt affected 

 by heat or the absence of heat, the one expanding it into dispersed and 

 incoherent particles, the other condensing it into the form of hail or 

 snow. By the absence of caloric which, at any temperature above the 

 freezing point, is sufficient to keep it in a state of fluidity. 



Objection 3. This would undoubtedly be the case were water raised 

 by the heat of 300° in usual summer heat, but this is not the case in 

 any part of the globe. Again, in all cases of evaporation we must make 

 allowance for currents of air which disperse the vapours and separate 

 its particles, counteracting its tendency to gravitate. 



Objection 4. This proves the effect of the condensation of the aqueous 

 particles by cold, but does not disprove their rarefaction by heat, which 

 would keep their particles divided, and thus prevent condensation. 



Objection 5. Here the agency of increased temperature in the sur- 

 rounding air renders the vapour invisible, as with our breath, which is 

 only visible when the temperature is below that of itself when it comes 

 from our bodies. The glass is raised on its surface to the temperature of 

 the surrounding atmosphere, and therefore does not condense the steam, 

 but were it taken out of a pail of ice, wiped dry, and immediately held 



