438 Prof. Waxuace on the Invention of the Pantograph 
The instrument has these two properties, by which its accuracy 
may be verified : When the zero divisions on the scales are at their 
indices, in which case, a copy made would be exactly the size of 
the original ; if two corresponding dots be marked anywhere on 
a surface, one by the tracer and the other by the copying pencil, 
and the point of the tracer be carried rondu and put on the 
mark made by the copying pencil, the latter should fall exactly on 
the dot marked at first with the tracer. Of course some allow- 
ance must be made for the impossibility of perfect workmanship 
in this as in other instruments. 2. If a straight line be drawn 
on paper, and there be laid off on it any number of equal parts ; 
then if an enlarged trace of the line, in some known proportion, 
be made by running the tracer along a ruler, and the divisions 
of the original be marked on the copy; this last ought to be 
sensibly straight, the divisions equal, and the original and copy 
ought to have the prescribed proportion. 
For obvious reasons, there is a limit not very remote to the 
power of making an enlarged copy ; that of making a reduced 
copy has, however, more scope. For two instruments, A and B, 
adjusted so as to make each a reduced copy, may be united by 
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