for di'Q'iving in PerJpeSfive, 569 



feiitatioii of the place and pofitloii of the line may be then 

 drawn upon the paper, by the top edge of the ruler ST, a trifle 

 longer at each extremity than it appears to be. This operatioii 

 may be repeated for as many lines as can be obtained in the 

 iirfl pofition of the papered board and parallelogram ; when they 

 nuifl: be fliifted into the other recefs and groove, to find the reft, 

 which may be now done without taking any further notice of 

 the divifions on the rulers. 



N. B. A common T fquare, applied to a board of this kind, 

 will anfwer mofl purpofes. For example : place the flock of 

 fuch a fquare in one of the grooves, having a blade not lefs 

 than the length PIK or HR ; mark the fpaces HI and QR 

 upon the upper edge thereof, and divide each of them into any 

 convenient number of equal parts, and figure the faid parts in 

 the ufual manner, to correfpond with each other, as may be 

 feen in fg. i . Now, fuppofe the flock of the fquare to be in 

 ED, it is plain, that all perpendicular lines may be drawn 

 upon the paper KLMN in their proper places, 'and (by means 

 of the divilions on the edge of the fquare) neaily of (though 

 properly a trifle more than) their true length. All the lines of 

 this defcription being obtained, the fhifting board mufl be 

 placed in its other recefs, and the ftock of the fquare into the 

 other groove ; then, beginning with the £rfl line, ^briiig the 

 edge of the fquare to agree with its limits, and mark them oft" 

 upon the line on the paper, and fo of all the refl in fucceflion 5 

 and join the points,- where necefTary, with a common ruler. 



FIG. III. (t a B. XIV.) 



.. This apparatus Ts contrived to avoid the trouble qf conti- 

 nually working againfl a board in a vertical pofition. Jij grd^r 



C c c 2 to 



