143 
tres are slipped on the pin and taken off, according to the 
degree of pressure required. At e there is a button with a 
wire pin descending from it, bearing on a small concealed 
lever, which, when pressed down, raises the pencil from the 
paper. This button is pressed by a finger whenever the 
pencil is to be moved, without marking the paper, g, call¬ 
ed the gallows, is made in the form as represented in the 
drawing, and fastened by pressing it into a socket on the T. 
Its height should be at least equal to the breadth of the 
drawing board. At h a pulley is suspended from it directly 
over the space between the two pullies on the T. i is a 
piece to slide into and out of the frame, and fastened in the 
inside by a wedge, k is an upright cylindrical piece, press¬ 
ed into a hole near the end of i. I is a horizontal piece 
made to slide up and down on £, and fastened to it at any 
height, by the screw m. n. is a flat piece of brass with the 
sight-hole in it, and will be the better if a ring be fastened 
to each side of it to exclude the light. At o is a weak wire 
spring rising from a cavity in the rod ; to this spring is fas¬ 
tened a fine smooth thread which is carried thence, through 
the groove on the rod, to, and under the first pulley on the 
T, thence up, to and over the puliey on the gallows ; thence 
down, under and beyond the second pulley on the T, to the 
end of the rod, where it is fastened, bearing a little on the 
spring at o. ft is a mark on the thread, which may be either 
a small conical bead, or the knot of a differently coloured 
thread. 
In the inside of the box there are apartments for the va¬ 
rious utensils of drawing, besides the parts of the instru¬ 
ment to be packed up in it. There is also a lid or top-piece 
to be laid on it and secured by a lock. 
From the construction of this instrument it is evident that 
as the pencil, with the port-pencil and rod, is moved for¬ 
ward, the mark on the thread will move upwards and vice 
versa ; and as the pencil with the T moves to the right or 
left, so will the mark. All these movements being on 
