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only affect drawings of very near objects, but in drawings 
of those that are at any considerable distance, it will be al¬ 
together imperceptible, and can be no objection to the gen¬ 
eral utility of the instrument. 
As a cheap, convenient and ready assistant to a person 
whose taste may lead him to the uoe of such instruments, 
this has its merits,which those who try it will duly appreciate; 
but I have never seen a description of any which can so com¬ 
pletely serve the purpose of drawing in perspective median - 
ically as the one of which I shall now give the description. 
THE DIORAGRAPH.— (Fig. 16J 
is a drawing board dropped into the surrounding frame, 
which is here represented in the form fof a box, to receive 
the other parts of the instrument when not in use ; b i c, c, 
is made in the form of the letter T, and is so called; d } d, 
are two pieces screwed on to the back of the frame, so that 
their ends join in the middle, and their upper edges form 
one straight line ; these are then called the rail. To the 
bottom of the T, near its ends, where the letters c, c, b , are 
placed, wheels are fixed to facilitate a lateral motion : The 
wheels at c, c, run in a groove on the rail, and the wheel at 
b on the front edge of the frame, e is a piece called the 
port-pencil, with two wheels to its bottom, moving in the 
grooves r, r. —From the port-pencil a piece is extended to 
J \ called the rod, in the under part of which is a groove, to 
receive the edge of a wheel fixed on the T, where the leg 
joins the crossing part; and immediately above are two 
pullies, one diameter apart from each other, sinking a little 
into a groove on the upper edge of the rod. To the right 
side of the port-pencil is fixed a flat piece of brass, with the 
two ends bent horizontally, having holes in them exactly 
over each other, through which the tube, or case that holds 
the pencil, is to slide, with little or no friction. On the top 
of the pencil is placed a socket, S, from which rises a pin ; 
circular pieces of metal, to serve as weights to give a due 
pressure to the pencil on the paper, with holes in the cen- 
