'4'^'4 Mdjor-Getjet'dl V^oy' ?, Account of the 



glafs rods were, for the degrees of difference of temperature^ 

 below that in which their refpedive lengths were laid otf, that 

 this fmall apparent difference of two- tenths of an inch, be- 

 tween the two modes of meafuring the thoufand feet, fhould 

 jiave been 0.17938 in. to have made the two refults exa^lly 

 -agree, which is a real difference of only 0.02062 of an inch. 

 Suppofing then every thoufand feet of the bafe to have been 

 meafiired by the chain with the fame attention, and confe- 

 quentiy with the fame, or nearly the fame fuccefs (and there 

 furely cannot be any reafon to doubt of the praclicability) we 

 Ihall have 27.404x0.02062 in. = o.565in. or a defeat of fome- 

 thing more than half an inch on the whole length of the 

 bafe. 



* When the length of the chain was laid off, the heat was 66''|, and that of the 

 glafs rods 68\ They will, therefore, only agree with each other accurately in 

 thefe refpeftive temperatures. The mean of twenty thermometers for the four 

 chain lengths of the forty-fixth hypothenufe gave a heat of 6i'^.6; and for the 

 fix chain lengths of the forty-fifth, the mean of thirty thermometers gave 59°.75. 

 The temperature of the 400 feet of glafs by the mean of forty thermometers 

 was 65°.3; and of the 60O feet, by the mean of fixty thermometers, it was 

 6o°.8. Now, from thefe data, and the expanfions of fleel and glafs, as deter- 

 mined by the pyrometer, the computation will Hand as follows : 



In. In. In. 



Steel / 4°^ 66.5-61.6=4.9 xo.03052=o.i4955'l_Q^^g^gr contract. 



l6oo 66.5-59.75=6.75x0.04578=030901/ '^ |ofiooo 



leet. 



]6oo 68.0—60.8 =27.2 X 0.03102=0.22334] " J "^^ ^°^^ 



"^ feet. 



Glafs/^OO 68.0-65.3=2.7 X0.O2o6Szro.05584') _Q^^^^3j-contraa. 

 (_6oo 68.0—60.8 =27.2 X 0.03102=0.22334] 



The 1000 feet of fteel fhould have contrafted more than! __ 



the 1000 feet of glafs, - - 



But the difference was found to be - - =0.20000 



H =0.17938 



Therefore the error of- the chain in tlefe<^ was - 0.02062X27.404=2 



0.565 in, or little more than half an inch on the whole bafet 



So 





