\QQ PARALLEL RULES, 



great height, fhaking and leaving fit for loading, a good cart 

 load in all; while, but for a wail at hand, my Shipham miner, 

 as ulual, defpifing novelties, would probably have been 

 wounded, having been with difficulty perl'uaded to take that 

 cover at four yards diftance. 



Thus, Sir, I have ftated what I take to be improvements in 

 this valuable art, and if they afford you or your readers any 

 gratification, I fliall not regret the trouble of putting them on 

 paper, being always, Sir, 



Your grateful reader, 

 G. C. 



III. 



Defcription of a nexv Parallel Rule, exempt from lateral Devi- 

 ation', inve/iled by Air. J. W. Boswell; zvith an Account 

 of the Imperfections of thofe alreadj/ made for the fame Pur» 

 pofe. 



To Mr. NICHOLSON. 

 Dear Sir, 



Inconveniencies X HE common parallel rule of four pieces has been long 

 deviation of"thc ^^"""^ inconvenient, on account of the lateral deviation of the 

 common parallel moving piece, which caufes a necellity of fliifling the pofilion 

 ^'^■^* of the whole rule frequently, when many parallel lines are to 



be drawn; that, befides the lofs of time which it occafions, 

 tends alfo to produce error in the parallelifm of thcfe lines. 

 It is notfuperior For this reafon it is in no refpecl fuperior to the more iimple 



p°anc\ndTu1e ^'^^PP^'^^*'^^ ^^^ '''^ ^^'^^ purpofe, formed by a triangular plane 



of wood or metal, moved along a common rule; and as this 



latter is more fteady, and ferves for other purpofes in draw. 



ing, it is preferred by feveral. 



Parallel rules as Many inftrumeats have been contrived to draw parallel lines 



opmtfwkhoit without being fubjea to the imperfe6tions here ftated; but all, 



I'lde deviation that I know of, are more or lets deficient in corre6tnefs, from 



are fubjcft to requiring an exadnefs in their formation hardly attainable, or 

 inaccuraey. r 



from extreme tendency to have this perfedion deranged when 



attained. 



The parallel rule 'The parallel rule with eroding conneftors, and two Aiding 



with niding joints, is fubjeft to both the above inconveniences. The leaft 



joints difficult to 



inakc exait, and P'^Y 



