BAR 



Whfrfa* M. De Luc's cxperhnents reduced, give this 

 qiiaiuity equal 1002.23 parts. General Roy compared a 

 nurcurial and an air lliermometcr, each of wliich was gra- 

 duated arithmetically ; that is, the units of the fcalcs were 

 equal bulks of mercury, and equal bulks of air. Their pro- 

 grefs it exhibited iu tlie following table. 



Table V. 



BAR 



212° ; and the fourth fliews the mean expanfion for eack 

 degree, 



Tahle VI. 



As equal increments of heat produce equal increments in 

 the bulk of mercury, the differences of temperature aie ex- 

 prtffed by the fecond column, and may be confidered as 

 equal ; and the numbers of the third column exprcfs the 

 fame temperatures with thofe of the firft. They direftly 

 exprefs the bulks of the air, and the numbers of the fourth 

 column exprcls tlie differences of thefe bulks. Thcfe are 

 evidently unequal, and they fhew that common air expands 

 moft of all when the temperature is 62 nearly. In order to 

 determine what was the aftual incrcafe of bulk by fome 

 known increafe of heat, general Roy took a tube of a 

 narrow bore, with a ball at one end. He meafured the 

 capacity of both the ball and the tube, and divided the tube 

 into equal fpaces, bearing a determined proportion to the 

 capacity of the ball. This apparatus was placed in a long 

 cylinder filled with frigorific mixtures or with water, which 

 might be uniformly heated to the boiling temperature, and 

 it was accompanied by a nice thermometer. The expan- 

 fion of the air was meafured by means of a column of mer- 

 cury, which rofe or funk in the tube. The tube being of 

 a fmall bore, the mercury did not drop out of it ; and the 

 bore being chofen as equal as pofTible, this column remained 

 of an uniform length, whatever part of the tube it chanced 

 to occupy. By this contrivance he was able to examine 

 the expanfibility of air of various denfities. When the 

 column of mercury contained only a fingle drop or two, 

 the air was nearly of the denfity of the external air. If he 

 wifli.-d to' examine the expanfion of air twice or thrice as 

 denfe, he ufed a column of 30 or 60 inches in length ; and 

 to examine the expanfion of air that is rarer than the external 

 air, he placed the tube with the ball uppermoll; ; the open 

 end pafiing through a hole in the bottom of the vefTel con- 

 taining the mixtures or water. By this pofition the column 

 of mercury was hanging in the tube, fupported by the pref- 

 fure of the atmofphere ; and the elafticity of the included 

 air was meafured by the difference between the fufpended 

 column and the common barometer. 



The following table fhews the expanfion of 1000 parts of 

 air, nearly of the cflmmon denfity, by lieating it from o to 

 212. The firft column fliews the height of the barometer ; 

 the fecond fhews this height augmeuted by a fmall column 

 of mercury in the tube of the manometer, and therefore ex- 

 preffes the denfity of the air examined in inches ; the third 

 contains the total expanfion of 1000 eqiul parts of air by 



If this expanfion be fuppofed to follow the fame rate that 

 was obferved in the comparifon of the mercurial and air 

 thermometer, we fhall find that the expanfion of a thoufand 

 parts of air for one degree of heat at the different interme- 

 diate temperatures will be as in the following table. 



Table VII. 



In order to have a mean expanfion for any particular 

 range, as between 12° and 92°, which is the moft likely to 

 comprehend all the geods^tlcal obfervations, we need only 

 take the difference of the bulks 26.038 and 222.006 = 

 195.968, and divide this by the interval of temperature, 80°, 

 and we obtain 2.4496, or 2.45, for the mean expanfion 

 for 1°. 



This table, which in its prefent form fhews the expanfi- 

 bility of air originally of the temperature o, may be eafily 

 adapted to a mafs of 1000 parts of air of the flandard tem- 

 perature 32°, by faying (for 212*^), 107 1. 718: 1484.210:: 

 1000:13489; and fo of the reft. Thus the following table 

 is conftrudted. 



Table 



