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Dr. Roget on the 



Fig. 6. 



Another ruler AB, of equal length to the former, being furnished 

 with a similar collar, is made to revolve round A, at the extre- 

 mity of the line A N, which is perpendicular to the axis N S. 

 The other ends, B and ?i, of these two rulers, are connected by 

 a third (equal in length to N A), which, during the movements 

 of N n and A B, preserves its parallelism to N A. The ruler 

 B n has a groove, running its whole length, in which a button, 

 projecting from the extremity of the rule S s, slides. The 

 ruler S.s is also furnished with a collar similar to the former, 

 which fixes it to the board at S, or at any other required point 

 in the line N S. A pencil, following the intersections of the 

 rulers N n and S s with each other, as they turn round their 

 respective centres N and S, will describe a magnetic curve. In 

 adjusting the several collars, care must be taken that the dis- 

 tances of A, N, and S from B, n, and s, respectively, are all 

 equal ; and, in order to effect this, it will be convenient to 

 have the rulers graduated to a scale of equal parts. The 

 greater these distances, the larger will be the curve described 

 by the instrument. When the ruler N n is brought down so as 

 to coincide with the axis N S, the other ruler S s is in the 



