CLIMATES OF GREAT MALVERN AND LONDON. 6i 



and the nature of the surface exposed : all low, damp places — grass 

 and raeadow-land — promote it in a peculiar degree; and the result- 

 ing visible forms of vapour, therefore, peculiarly affect such locali- 

 ties, which are really colder than more elevated places — light misty 

 fogs, producing a sensation of chilliness, hang upon them at night 

 and early in the morning, and all the noxious influence cf malaria 

 is concentered there. 



The fogs which form at night over valleys and low situations, 

 and heavy dew, are therefore objects which the meteorologist should 

 not fail to notice ; for they are caused by the precipitation of aque- 

 ous vapour : and it is more than probable, if this vapour has been 

 the carrying agent to any malaria, that its development, in a 

 deleterious form, at the same period commences. 



The aqueous vapour always permeating the permanent elastic at- 

 mosphere, possesses the same mechanical properties as it would ex- 

 hibit in a separate state ; the two fluids exercise only a certain 

 degree of opposition to each others motion. Vapour is constantly 

 tending towards the point where the elasticity is least, and its par- 

 ticles will very often be permeating the interstices of the perma- 

 nently elastic fluid, in a direction opposed to the wind. If the flow 

 of vapour is slow and steady, this motion, in totally opposite direc- 

 tions, may continue ; if, on the other hand, in places not far re_ 

 moved from each other, the temperature of the dew point, or, in 

 other words, the elasticity of the vapour, is very different, then its 

 sudden rush will carry the permanently elastic fluid with it, and the 

 wind will h\ow in the direction where the (Jew point, or the elasti- 

 city is least. Thus, during hot and very dry weather, when the 

 air is nearly calm, dark, heavy clouds sometimes form, and rain de- 

 scends in torrents ; immediately the wind springs up — for the elas- 

 ticity of the vapour evolved from the contact of the rain with the 

 dry and heated ground is much greater than that in the neighbour- 

 ing higher regions — and it rushes rapidly towards them, carrying 

 with it the permanently elastic fluid ; so that the wind hlowsjrom 

 the point where the rain has fallen. But these are only very li- 

 mited phenomena — thunder-storms, and the direction they impress 

 upon the motions of the air, being very partial. What we wished 

 to remark upon, was, that a current of vapour and possibly, also, a 



