438 Prof. Wallace 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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