of the Movements of the Barometer, 8fc. 15 



moon, would be to produce atmospheric tides, similar to those 

 observed in the sea, varying the weight of different columns of 

 the atmosphere by augmenting and diminishing their height 

 These tides would doubtless, too, be measured by our barometers. 

 They are obscured, in the existing state of things, by causes 

 modifying the elasticity of the lower region of our atmosphere, 

 and so cutting off, for a time, the influence of the height and 

 weight of the superincumbent columns. 



These regular movements of the atmosphere are disturbed 

 chiefly by the irregularities on the earth's surface, by which the 

 winds are diverted from their original and natural course, — being 

 divided asunder by mountains, and made to clash violently together 

 in the course of valleys and rivers. These effects are followed 

 by others which arise out of the admixture of portions of air of 

 different temperatures, and containing different portions of aqueous 

 vapour; and it is not improbable that there are other phenomena 

 of an electric nature, not hitherto fully appreciated and understood, 

 which act silently and obscurely in general, though they are occa- 

 sionally manifested in the phenomena of thunder and lightning. 



The object of this paper is not to treat fully of the movements 

 of the Barometer, but to point out one of the causes of these 

 movements more particularly than has hitherto been done. This 

 cause is the transition of vapour in the atmosphere from the trans- 

 parent to the opaque and fluid form, — or the formation of clouds, 

 and of rain. We shall now, therefore, proceed to trace the effects 

 of this change upon the elasticity of the atmosphere in the region 

 of the clouds, and consequently upon the barometer. 



It is well known how much the presence of water adds to the 

 expansibility of the air in contact with it, by heat. In the 

 experiments of Guyton and Duvernois*, the object of which was 

 to ascertain the comparative expansibilities of the different kinds 

 of gas, the presence of an inappreciable quantity of moisture 

 ioned an error of such magnitude, that, of gases which are 

 now known to expand alike, some appeared to dilate five times as 

 much as others by the same application of heat. 



* Journal de VEcole Poly technique, Cap. 2. + Enai sur C Hygrometrte. 



