[ 109 ] 



To equate a ghett' Column of Mercury in the Barometer fo as to. 

 a/certain its proportionate Length to a Column 0/2,0 Inches with 

 a Temperature of SS^ o/' Farenheit's Scale. 



As the mercury in the barometer expands and contfad's itfelf 

 in proportion to the heat and cold of the atmofphere, fo that the 

 fpccific gravity of the metal is in fadt different at different times, 

 it is evident that the adual height of a column of the atmofphere 

 being given, if the temperature of the air as to heat arid cold' 

 varies, the length of the rod of mercury fupported by fuch a 

 given column will be longer or fhorter in proportion to the- 

 greater or lefTcr degree of atmofpheric heat. To afcerfain ttiefe- 

 fore the variations in columns of mercury of different lengths, 

 arifing from the different temperatures of the" atmofphere, fome 

 given length and given temperature muft be fixed on, as the 

 term or flandard of comparifon. 30 inches of mercury, and 55*' 

 of Farenheit's thermometer, have been generally chofen for this 

 flandard, as the one is pretty nearly the mean height of the 

 mercury in the barometer at the level of the fea, the other the 

 point of the fcale of Farenheit's thermometer, ufually in thefc 

 climates marked temperate. If either of both of thofe terms vary, 

 the obferved length of any column muft be equated to reduce it 

 to what it would be if the mercury flood at 30 in a barometer at 

 the level of the fea, and the thermometeriat 5 5°. It has been proved,, 

 by very exad and repeated experiments, that the barometer,, 

 flanding at 30 inches, the expanfion produced in the whole 

 column by a change of one degree- of heat in the thermometer, 

 is equal to ,00304 of an inch. On this calculation, for a 

 variation of 33" of the thermometer above or below 55'' you 



muft 



