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difcover fome general circumftance in which a known prefTure, a^ 

 known temperature, and a known weight may be afcertained as 

 nearly as phyfical refearches admit or demand. Now this known 

 and general circumftance is the temperature of the congelation of 

 water, or 32'' on Fahr. thermometer, or o of Reaumur, and is in- 

 variable all over the globe. The known prejfton is that of 30 

 inches Englifh of mercury, whofe fpeclfic gravity in temperature 

 6o* or 62*' is 13,6 nearly, on a bafis of one fquare inch Englifh 

 meafure ;. a preffion which may be univerfally afcertained, as mer- 

 cury may be eafily purified fo as to poffefs that fpecific gravity, at 

 leaft very nearly, and minute precilion is in moft cafes unneceffary, 

 at leaft in meteorology. And, laftly, the knownjw^^r^' is 87 feetj 

 or 1044 inches, or 14,5 Englifh fathom, this being thequantity of 

 air which in this circumftance weighs as much as one-tenth of a 

 cubic inch of mercury, that. is, 344,32 grains at the level of the fea, 

 at which the general mean height of the barometer is 30 inches. 



This determination pofTefTes this peculiar advantage, that if the 

 preffion 'j(7r/<?j-, the temperature 32° remaining unaltered, (as- when 

 the barometer ftands- above or belov/^ 30 inches) the meafure or 

 number of fathoms of air that poffefs this weight may be difcovcred 

 by the table of common logarithms,. rejeding the charaderiftie ; for 

 if the barometer ftands at any given height, then fubftrading from. 

 Ae four firft figures on the left of its logarithm, the four firft 



figuresi 



