277 
ABCD is a Drawing-Board, ZSX a bar of wood fixed to 
the centre of CD and flush with the Board M,H,TH,M, a 
straight bar sliding on the Board through the fixed guides H,, H,, 
so that TQ a ruler, fixed at right angles to Mf, MV, at its central 
point 7, sweeps over the Board and is always parallel to the 
edges AC or BD. 
oz { apa aT) RUG oes Ms 
a a 
A, 
el 
xX 
Sis a small ring fixed at any required pointin ZX. £ is 
another, fixed at any required point in the Board. Any point 
Q in the ruler will trace out a line (as QZ) parallel to CD. 
ELS is perpendicular to QZ. EL (drawn to seale) is the height 
of the eye above the given Plan, ZS the distance from the plane 
of reference. 
