Mr. Peacock's Defcnptions, Sec, ^6y 



the fame dlmenfions as the field of view refpe6liv^ely. OP a 

 fleel fliding-piece, equal in length to the diflance KF Or IN ; 

 at the lower end P is a ileel arm terminating in a point; and at 

 the upper end, at O, is a fimilar arm, terminating with a brafs 

 button, in the center of which is a fharp fteel pricker ; the faid 

 pricker and the point P are to be equi-diftant from the edge of 

 the blade of the fquare : this arm O is to have the faculty of a 

 fpring, in order that the pricker may clear itfelf of the furface of 

 the paper as foon as the finger quits the button, in the fame man- 

 ner as is ufual in the apparatus of large protradors. This flidinp- 

 fteel-piece may be drawn out of the dove-tailed or rebated 

 groove at pleafure, and the T fquare will then be fit for ordi- 

 nary ufes. 



To ufe the Injlrument, 



Having fixed the board truly level and perpendicular, and / 

 placed the point of fight, or hole for vifion, at fuch a height and 

 diftance as fliall be produ6live of the befl: effe£l, move the 

 fquare with one hand, and the fleel Aider with the other, until 

 the point P coincides with the eye and any point or angle in 

 the original obje£t. Prefs the pricker at O, and the puntcure 

 will be the true place, or copy, of fuch original point or angle, &c. 



N. B. All perpendicular lines may be drawn at once (in 

 pencil), by bringing the left-hand edge of the fquare to coin- 

 cide with the original line and the eye ; and their lengths may 

 be very nearly determined by the graduated edge of the fquare, 

 fo as to prevent confufion from unneceflary lengths of lines. 

 The faid graduated edge will alfo give the points in all curved 

 or irregular objedts. 



Vol. LXXV. C c c f i g.» 



