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to the curious, and fhall only add, that from continued and 

 cautious experience it is manifeftly certain that thefe barometers 

 are as fenfible and fliew the fmalleft changes in the weight of 

 the atmofphere as accurately as thofe whofe cifterns are adually 

 open, and that I have tried them repeatedly againftfome of the 

 ingenious and accurate Mr. Ramfden's provided with floatins; 

 guages, verniers, &c. and the refults have never varied two inches 

 from each other in altitudes of above three hundred feet. 



On Barometrical Menfuration. 



The inftruments required for this purpofe are two good 

 barometers of a proper and fimilar conftrudion, with two 

 thermometers of Farenheit's fcale to each, one attached to the 

 barometer and covered in its cafe as near the mercury as poffible, 

 to determine the adual heat or cold and the confequent expanfion 

 or contradion of the mercury in the inftrument. The other 

 detached for obferving the temperature of the air in the fliade, 

 and from thence to deduce the value in length of a column 

 thereof equal in height to a given column of mercury ia the 

 barometrical tube. 



i^ If the heights to be meafured are but fmall, and the 

 different ftations acceffible in a fhort interval of time, one ob- 

 ferver by going from place to place may determine them with 

 fufficient accuracy ; but if either the heights or the intervali 

 of the nations in diftance make a confiderable portion of time 



^ 2 ncceffary 



