,v 



Geological Olfervations. 279 



gree of heat of each being equal, or reduced 

 to an equality, and the common temperature 

 of the atmofphere being given. Two baro- 

 meters are therefore ufed, and each has a 

 thermometer annexed to it. The degree 

 of heat to which both barometers are re- 

 duced, is 55? of Fahrenheit, yet if either of 

 the barometers be at 30 inches, and the ther- 

 mometer attached^ to it at 55, no reduction 

 need be made in that barometer j but if either 

 barometer be at 30, and the thermometer at- 

 tached to it below 55?, we muft add the expan- 

 fion the mercury would have by the heat of 55?, 

 or if it be above 55? we muft fubtrad the excefs 

 of expanfion it gains by that heat.* Now every 

 degree of Fahrenheit produces an expanfion, of, 

 00304 of the barometrical inch, when the baro- 

 meter is at 30, therefore when the thermometer 

 is at n degrees below or above 55? we muft 

 add in the former, or fubtradt in the latter cafe, 

 0,00304 n to, or from the barometrical height. 

 But if the mercury in the barometers ftand 

 above or below 30 inches, then let the cor- 

 redion be found as if the mercury were at 

 30, and let it be denoted by r, the correftion 

 fought by x and the barometrical height b^ 



then ~= x. The thermometers fhould go 



alike, or if not, the difference fhould be ob- 

 ferved and fubtrafted. 



* Thefra&ions to be added orfubtra&ed, are marked on 

 3 feparate fcale, called the fcale of corrcdtion. 



The 



