XXI 1 . Sketches and Defer iptions of three fimple Inftruments for 
drawing Architecture and Machinery in P erfpeCtive. By Mr . 
James Peacock; communicated by Robert Mylne, Efq . F.R.S. 
Read March ip, 1 785. 
...i i ■> ; . • / , •. . ... . ■ 
S OME of the following machines mull be placed upon the 
front edge of the table upon which they are to Rand. 
The fights may befupported by a three-legged Raff. 
The Rocks of the fiquarcs or indexes may have Reel fprings 
upon their edges, in order to keep them in any affigned part of 
the grooves in which they are to Ride. 
FIG. I. (t A B. XIII.) 
ABCD a drawing board, to be fixed on a table or Rand, See . 
in a vertical politico. AB a Riding-piece for the top of the T 
fquare, having a rebate therein to form a groove, as exprefled 
by the dotted line. CD, Riding-piece for the bottom of the fquare, 
Raving a rebate therein to form a groove for the reception of 
the Rock as deferibed by the dotted line ; this Riding-piece to 
be of fufficient length to receive and fupport the faid Rock 
when the blade of the fquare is coincident with the lines 
KNFH or LNGI. E a hole to receive the arm or Rider of the 
fight-piece, to be conRruRed in the ufual manner. FGHI an 
opening forming the field of view for the prototype. KLMN a 
iheet of paper fixed on the upper part of the board for the 
; copy> the four inner lines whereof form and inclofe a ipace of 
the 
