MARINE BAROMETER. J^Q 



drawji from the sliore to the top of the highest land : but, almost 



immediately, the succeeding bodies of air, at a distance fiom the and infciences 



shore, will feel the effect of the obstruction ; and being im- '" ascertain the 



pel'ed by those tliat follow them, will begin to rise, takmg changes of 



their coarse for the top of the highest land, before they como vvina ,nd wea- 



to the shore ; by which means, the stratum of lowerair will be '^"^'^J^^jr^gj. 



deeper between the top of the land and the shore, and to some change in the 



distance outfrom it, than it is either upon the mountains or in i^i^'in® baro- 



T- t 1 meter, 



the open sea. It this is admitted to be a necessary conseqience 



of a wind blowing upon the shore from the sea, it follow., tnat 

 the mercury ought to stand something higher when such a w.nd 

 blows, whether it is from the south or any other quarier, than 

 it will with the same wind where it meets no such obstruction ; 

 and the more direct it blows upon the coast, and the hgher 

 the land is, (all other circumstances being equal,) the higher 

 ought the mercury to rise. On the other hand, when the wind 

 comes from off the hills, this dead and dense air will be dis- 

 placed, even from its hollows under the highest land ; both oa 

 account of its own expansion, and because its particles will be 

 attracted by those of the air immediately above, which are 

 taking their unobstructed course out to sea; and thus the air 

 over the coast will resume its natural state with a land wind. 



In order to appreciate duly the effect of sea and land winds 

 upon the barometer, in the preceding examples, it is neces- 

 sary to be recollected, that in the southern hemi-phere, a wind 

 from the south has a natural tendency to raise the mercury in 

 the open sea, and one from the north to depress it ; probably, 

 from the superior density of the air brought in by the former; 

 therefore, if the mercury rises quicker and hight;r with a south 

 wind upon the south coast, then it does with a north wind upon 

 the north, it is not to be at once concluded, that the effect of 

 the wind as coming from the sea, is less upon the north coast; 

 for it has, in the first place, to counteract the tendency of the 

 mercury to fall with a north wind ; and, in some cases, its effects 

 as a sea wind may be as considerable, relatively to the latit' de, 

 where there shall be no rise in the barometer, as upon the south 

 coast it might where a considerable one took place. The same 

 thing maybe said of the winds from the east and from the west ; 

 lor where the vicinity of land is out of the question, the former 

 generally causes an ascent (from what principle, I leave others 

 to determine,) and the latter a descent in the barometer, and T 



S 2 believsi 



