20 Dr. Hall on one of the Causes 



three-fourths at Marseilles, whilst it varied only one line and 

 three-fourths at the top of St. Pilon, which is 960 yards above the 

 level of the sea. 



; I must observe, in this place, that I am fully aware that it may 

 be objected to the preceding observations, that the quantity of 

 water in the atmosphere is not sufficient to account for the move- 

 ments of the barometer, and for winds, from the changes in its 

 state from transparent vapour to that of clouds or rain. To this 

 objection I would reply, first, that it is the object of this paper to 

 propose this, only as one cause of the movements of the barometer, 

 and of wind, whilst it is admitted that there may be, and undoubt- 

 edly are, many others ; and secondly, that the circumstances at- 

 tending these changes in the state of the atmospheric moisture, 

 and all the effects of these changes, are probably not yet fully 

 understood and appreciated. Two points, at least, have been 

 established : — the first, that there is an unequivocal connexion 

 between the fall of the barometer, the production of wind, and the 

 formation of clouds or rain ; the second, that the transition from 

 the elastic to the fluid state of the atmospheric moisture does, in 

 part y at least, account for these phenomena — must be attended by 

 a certain diminution of bulk and elasticity — and consequently by a 

 movement in the adjacent regions of the atmosphere, and by a fall 

 in the barometer. The principles here pointed out may obtain in 

 the minor but more usual changes in the atmosphere ; whilst the 

 more extraordinary phenomena of gales and tempests may, and 

 doubtless do, involve many other. Our atmosphere is so compli- 

 cated a structure, that no simple theory will ever explain the 

 whole of its phenomena. It has just been suggested, that as all 

 the causes of the movements of the barometer are not hitherto 

 ascertained, so all the circumstances attending the precipitation 

 of the atmospheric vapour, assumed as one of these causes, are 

 probably not yet fully appreciated: — one effect of the condensa- 

 tion of the atmospheric moisture, is an actual diminution of the 

 weight of the atmosphere, and this would operate on the baro- 

 meter, were the elasticity of the air unaffected ; but as the vapour 

 really constitutes a part of the atmosphere, and adds to its volume, 



