On the Barometer. 3 77 



The particular or more immediate indication of the wea- 

 ther which i3 coming, arises from the alteration which is 

 taking place in the density of the atmosphere, and which 

 the barometer exhibits by the rising or sinking stale of the 

 mercury ; the weather becoming comparatively clearer at 

 the atmosphere is becoming denser, and duller as the atmo- 

 sphere is becoming rarer*. 



Eence, if the barometer were as portable and as con- 

 venient for reference as a watch, we should seldom be at a 

 loss to know, at least for short intervals, what kind of wea- 

 ther was coming f. 



The ordinary~range of the barometer in this climate is 

 from rain to fair; rising however, occasionally, as high 

 as settled fair; and sometimes, though very rarely, as 

 high as very dry: and sinking, occasionally? as low as 

 much rain; and sometimes, though very rarely, as low 



as STORMY. 



It is scarcely necessary to observe that north and east 

 winds, in consequence of passing to us from a colder cli- 

 mate, and over land, bring a denser, colder, and dryer at- 

 mosphere ; and south and west winds, coming to us from 

 a warmer climate, and over the sea, bring a rarer, warmer, 

 and damper atmosphere; and moreover, that the capacity 

 of air for retaining water in a state of chemical combination 

 is increased by coming from a colder to a warmer tempera- 

 ture ; and diminished, by coming from a warmer to a colder 

 temperature. 



It must be equally apparent, that the greater or less ele- 

 vation of the clouds depends upon their own degree of 

 density, and that of the atmosphere which supports them. 



With regard to the immediate causes of the direction 

 and changes of the wind in this climate, I consider them 

 as involved in too much obscurity and uncertainty to say 

 any thing satisfactorily about them ; and with respect to 

 electricity, which though doubtless a powerful agent in 

 meteorological effects, 1 consider it rather as a matter of 

 curious speculation than of practical utility. 



I have therefore only to add, that by a due consideration 

 of the causes enumerated above, connected with the more 



* The difference that might be supposed to arise in the height of the 

 barometer from the effects of different degrees of heat on the atmosphere, 

 may in observations of this nature be entirely disregarded, g these effects be- 

 ing very nearly equalized by the expansion and contraction of the mercury 

 in the barometer, from the same cause. 



f As the atmosphere is almost constantly varying in its degree of den- 

 sity ; so is the barometer, which is an accurate measure of its density, as con- 

 ttantly varying in its altitude, and should therefore be frequently referred to. 



obvious 



