Description of a Perspective Instrumen t . 261 
“ Perspective is not, however, recommended merely as 
a means of improving the taste, but as it is useful in faci- 
litating the knowledge of mechanics. When once chil- 
dren are familiarly acquainted with perspective, and with 
the representations of machines by elevations, sections, 
&c. prints will supply them with an extensive variety of 
information; and when they see real machines, their 
structure and use will be easily comprehended. The 
noise, the seeming confusion, and the size of several ma- 
chines, make it difficult to comprehend, and combine 
their various parts, without much time, and repeated 
examination ; the reduced size of prints lays the whole at 
once before the eye, and tends to facilitate not only com- 
prehension, but contrivance. Whoever can delineate 
progressively as he invents, saves much labour, much 
time, and the hazard of confusion. Various contrivances 
have been employed to facilitate drawing in perspective, 
as may be seen in “Cabinet de Servier, Memoirs of the 
French Academy, Philosophical Transactions, and late- 
ly in the Repertory of Arts.’* The following is simple, 
cheap, and portable. 
“ Plate 7, fig. 2, A, B, C, represent three mahogany 
boards, two, four, and six inches long, and of the same 
breadth respectively, so as to double in the manner re- 
presented. Fig. 3, the part A is screwed, or clamped to 
a table of a convenient height, and a sheet of paper, one 
edge of which is put under the piece A, will be held fast 
to the table. The index P is to be set (at pleasure) with 
its sharp point to any part of an object which the eye sees 
through E the eye -piece. 
“The machine is now to be doubled as in Fig. 3, tak- 
ing care that the index is not disturbed ; the point, which 
! was before perpendicular, will then approach the paper 
horizontally and the place to which it points on the pa- 
per must be marked with a pencil. The machine must 
