^-»^ • M^ij&f 'General RoY*3 Account of the 



'Expanjion of the Deal Rods, 



It has been an opinion generally enough, although, i^ we 

 have feen, erroneoufly received, that very ftraight-fibred 4eal 

 vz-as not at all, or but little, affe6led longitudinally by the; hu- 

 midity of the air. That we might not be led aftray by truft- 

 ing to fallacies of this fort, the flandard rod had been. provided; 

 which benig always clofely fhut up in its cheit, except during 

 the fliort interim of comparifon, could feel but a fmall propor-. 

 tion of the effeds which the meafuring rods fuffered, thejfe: 

 being conftantly expofed to the open air throughout the day, as, 

 well as to the moifture of the night, when lying under the. 

 oil-cloth canopy-. The ftandard rod, it is true, coi^Id not be- 

 accurately compared wdth the brafs fcale : for although when 

 conftru6led, . brafs pins, forty inches afunder,^ had been driven: 

 into its ftem, forthe purpofe of fuch comparifon, yet thefe 

 had afterwards been difplaced, or at leaft the points upon thetn 

 dejfaped, by the planing over of t;he, upper furface. This cir-. 

 cumflance, which was unattended to when the operations! 

 cpmmenced, isino^;of no -cqnfequeiice ; becaufe> from an ex- 

 periment hereafter to be mentioned;, the length e;uilg of the flan-: 

 di^rd may be pretty nearly afcertairied. But UnzQ there are fome 

 contradictory circumftances, foon to be mentioned, in the ope- 

 ration with the deal rods, which would have^ made a repetition, 

 •of it abfolutely neceffary, if^ we had not now obtained thofe of 

 a different kind, fo very ynexc-eptionable in thbir nature and 

 mode of application, as, in tj^e^ |)v.e.(eul;;. cafe, to admit of no 

 competition between the two refults, and to render it improper 

 on our part ever to have farther recourfe to the firfl; fo there 



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