Mr. Peacock’s Defer options. See. 367 
the fame dimenfions as the- field of view refpedlively. OP a 
fleel Hiding-piece, equal in length to the diflance KF or IN ; 
at the lower end P is a fleel 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 Heel pricker ; the faid. 
pricker and the point P are to be equi-diflant 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 Qf 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 protradlors. This Hiding 
fleel-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 Infrument . 
Having fixed the board truly level and perpendicular, and 
placed the point of fight, or hole for vifion, at fuch a height and 
diflance as fhall be produdlive of the befl effedl, move the 
fquare with one hand, and the fleel Aider with the other, until 
the point P coincides with the eye and any 1 point or angle in 
the original objedt. Prefs the pricker at O, and the pundture 
will be the true place, or copy, of fuch original point or angle, See. 
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 unneceffary 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. 
