construction. 



1(8G INSTRUMEHT FOR REDUCING OR El^IiARGING PLANS. 



of the larger, and the 192 Hnes*remaining give me 64 fa- 

 thoms each reprepented by 3^ lines*. 



I afterward subdivide each of these scales by three. 

 »nd using it. j fi^ together the plan and the paper on which it is to be 

 reduced, the latter being under the small scale ; and place 

 the rule so that it can traverse circularly as much of the 

 large plan as its extent will admit. The rule being fixed on 

 the large plan wherever it touches a point to be transferred 

 to the paper, I note the number of toises on the large scale, 

 and opposite the same number of toises on the small scale I 

 make a mark with the point of a needle set in a handle, or 

 merely with a fine lead pencil. Thus I set down all the parts 

 of the plan one after another, which are found just and in 

 due proportion. 



If the plan to be reduced exceed the length of the rule, 

 the instrument may be removed to another place. I need 

 not mention the necessary precautions in this case for 

 placing the minudometer properlyf. 

 A different At first I placed my pivot between the two scales, count- 



ing the divisions in opposite directions; but as the plan was 

 reversed in this case, I had not the advantage of comparing 

 it readily with the original as I proceeded. 



It is obvious, that, if we would have other divisions, we 

 must have different rules, or trace these divisions on paper, 

 and paste it on the same rule. The rule may be graduated 

 also on both sides J. 



V. 



♦ It might be supposed from the text, that Mr. de la Chabeaussicre 

 began to count the divisions of tlie large scale from this point; but this 

 would be obviously wrong: both scales must begin t.h'^ir count from the 

 ]>ivot, consequently the first division in the larger scale must be reckoned, 

 in the instance before us, as SS, so that both scales will end with OG. It 

 should have been said too, that the pivot, or the hole for it, must be 

 placed in a line with the edge oi the scale carrying the divisions, C. 



f Though the divisions of the scale amount to 96 toises, there are only 

 64 that can in reality be used. Consequently it must be necessary to shift 

 thfe nain udometer and the paper once at least. C. , 



•\\£ the edges were bevilled in opposite directions, and the rule were irt 

 two parts, made to hi into each other either way where the smaller scale 

 terminates ; and the units were of different lengths, though similarly di- 

 yided ; this would give four proportions for diminishing or enlarging. If 

 f9r instance the principal divisions of one of the large scales were an inch. 



