BOOK III. C H A P. VII. <^59 



(lall mentioned) the mercury is rarely known to rife above 30 Inches, and. 

 its fettled ftate is about 29 }. Jamaica, lying nearer to the North Ame- 

 rican continent, and being a much larger ifland, is more fubjedt to vari- 

 able winds ; and the Norths, which always raife the mercury higheli, 

 are far ftronger here. The mercury is therefore not fo ftationary here a^ 

 in the Caribbee iflands, which lie near the Equinodlial. The atmofphere 

 is heavier in its prefTure ; and would be more falubriqus, for this reafon, 

 to European conftitutions, if the woods were opened. We may afcribeto 

 thispreflbre of the air, in great meafure, that lively flow of fpirits which 

 moft of tlje inhabitants enjoy, which may naturally happen whereever 

 the mercury's ftation is ufually near about 30 inches. The variations here 

 are minute, as DoAor Halley has remarked, except upon the approach of 

 ftormy weather ; and hence the preflure of the atmofphere on the body is 

 generally uniform, and equable, from day to day, or, at leaft, with very 

 little fenfible difparlty. 



Yet no contemptible advantage may be derived from this inftrument 

 even here ; where, although the mercurial motions are fmall, yet they 

 afford certain indications of change in the weather likely to happen in 24 

 hours. A knowledge therefore deducible from fuch a prognoftic is not 

 without its ufe to the planter, in direding his operations in the field, with 

 regard to planting or fowing. 



Science too may be benefited from its application in a different way. 



An accurate meafurement feems to be wanted of the height of the dif- 

 ferent mountains, particularly the various Blue Mountain ridges. 



In taking the height of a mountain with the portable barometer, the 

 experlmentor is firfl to remark, at what degree the quickfilver ffands at 

 the level of the fea, or (that being too diftant) at the foot. 



Having afcended to the fummit, or as high as he chufes to go, he is then to 

 obferve how many inches, and parts of inches, the mercury has fubfided. 



From variety of experiments it has been fuppofed, that the mercury 

 fubfides one tenth of an inch at every ninety feet of afcent ; but It has like- 

 wife been proved, that, from the different degrees of denfity in the air, 

 the higher we afcend, if the mercury falls one tenth of an inch at the firft 

 ninety feet of rife, it will not fall the next tenth, till the barometer is 

 carried up ninety-three feet higher; and fo the height of every column 

 of air, of the weight of one tenth of an inch of mercury, will vary ac- 

 cording to the height of its fituation in the atmofphere, each being about 



4 P 2 thre^ 



