12 



ON THE SEPABATE PRESSDBE8 OF THE AQUE0U8 AND 



surface, notwithstanding that their proper pressure has been 

 made positively less by the wanning and lightening influence 

 of condensation. 



The local and particular causes of these small changes may 

 be best traced, when they occur with such constancy and 

 regularity as will enable us with tolerable certainty to 

 connect each change with the cause that produces it. The 

 daily fluctuations of the barometer, and the occurrence of 

 daily sea breezes in certain parts, afford therefore the best 

 means of studying the subject, but there seems no reason 

 to doubt that the same causes, whatever they may be, 

 which are in operation in those parts, although modified 

 by circumstances, produce the more extensive and irregular 

 alterations which are found to take place in many parts of 

 the world. 



The general gaseous atmosphere, when at rest, has an 

 equilibrium of pressure at the surface of the sea: it would 

 therefore be the same in all parts of the world, if the 

 temperature were everywhere the same at the same level. 

 And those changes of temperature, in the mass of the atmo- 

 sphere, which are produced directly by solar rays, are so 

 gradual and slow as to enable such a highly elastic body as 

 the atmosphere readily to re-establish the equilibrium after 

 it had been disturbed by direct solar heat. The gaseous 

 pressure, therefore, would be about the same over every 

 part of the globe at the level of the sea, were there 

 not some other disturbing cause that operates locally; and 

 that there is such a cause, is sufficiently indicated by the 

 local and temporary changes of the barometer that take 

 place in various parts. Attempts have been made, not 

 entirely without success, to trace iso-barometric lines over 

 the globej and if we had them in considerable numbers 

 that could be relied upon, they might materially assist in 

 oujr inquiries. 



