for drawing in PerfpeSlive. 371 
the correfponding fide of the board on your defk or table, and 
draw the line of its precife length and pofition. 
N. B. If this is thought too operofe, the brafs groove and 
Hiding pieces M, N, may be rejefled, and the blade of the 
fquare may be graduated on one or both of its edges at pleafure ; 
and all lines in the fame direction may be drawn thereby exactly 
as to their pofitions, and nearly, though fomewhat exceeding, 
their lengths * ; and their precife lengths may be determined at 
the fame time the lines in the contrary pofitions are drawn, 
whofe lengths will be given at the fame time by the lines firfi: 
drawn. 
* This will be effe&ed, by noticing the numbers upon the blade, and taking 
thofe next beyond the apparent limits of the line j and by this means the drawing 
will advance without theleafl: confufion. 
