370 Mr. Pe a cock*s Defcrlptions of Infiruments 
to this, two fquare boards are to be provided, equal in fize, and 
of fimilar conftrudtion ; one is to be fixed in a vertical pofition, 
for viewing the original objedt through a proper aperture ; and 
the other is to be laid flat upon a defk or table, for the greater 
eafe and conveniency of drawing the copy upon paper to be 
fixed thereon for that purpofe. 
ABCD is the vertical board ; EFGH the opening therein, 
forming the field of view j IKL the T fquare, the blade 
thereof PL being moveable about the center P, with a mode- 
rate degree of ftiffnefs ; the flock K is to Hide in a rebated or 
dove-tail groove AD, and be fixable to any part thereof by the 
fcrew O ; the fleel points MN are to move with moderate eafe 
in a rebated or dove-tail brafs groove in the middle of the 
blade of the fquare ; upon the back of the groove AD are to be 
fixed two brafs pins QQ, to reft in proper holes, fimilar to 
the holes marked RR; and the fame kind of holes are to be made 
in the corner of the board whereon the copy is to be made. 
To ufe the Machine. 
Having placed the board ABCD in a truly vertical pofition, 
fix the fhifting groove AD in the rebate, on the moft conve- 
nient fide of the board, by means of entering the pins Q^into 
the holes R ; then loofen the fcrew O, and move the flock 
IK, and at the fame time turn the blade PL upon its center P, 
until one of its edges fhall be coincident with fome original 
line ; then fix the flock by turning the fcrew O ; move the 
points M and N, till they exadlly include the apparent length of 
the faid line ; then take off the fhifting groove AD, together 
with the T fquare or bevii fixed thereto, and apply the fame to 
the 
