318 PROCEEDINGS OF THE AMERICAN ACADEMY 



C. S., for Hy^^ R^ ^ and R^"^ of the m£ter, and the corresponding 

 defining lines of the yard, is nineteen minutes. The average time of 

 a single comparison of two units is therefore somewhat less than two 

 minutes. 



{!).) Comparison by means of two Microscopes attached to the same 



Movable Carriage. 



The steps are as follows : — 



(1.) The standards are placed in position upon the table S, one 

 near the middle, and the other at a given distance in front of it. 



(2.) The microscope K occupying a position near the middle of the 

 comparator, the defining line at the left end of the bar is brought into 

 focus of the microscope M by the screw M^ Fig. 7. The carriage is 

 then moved by the handle D till the defining line at the right end 

 of either one of the standards is in the field of the same micro- 

 scope. The adjustment for parallelism with the cylindrical ways is 

 now made with the lever S*"' and for focus with the screw S''. 



(3.) Both microscopes are now placed upon the plate K. By 

 means of the adjustments shown in Fig. 7, microscope M is adjusted 

 upon the left-hand defining line of one bar, and M^ is adjusted upon 

 the corresponding defining line of the bar in front of it. 



(4.) By a motion in translation through D, the right defining line 

 of the first bar is brought into the field of M, and adjusted for coinci- 

 dence with the fixed micrometer wire by the lever S''. 



(5.) The difi'erence in the length of the bars is then measured with 

 the micrometer of Mj. 



(G.) The bar in front is now placed at an equal distance back of 

 the first bar, and the operation described above is repeated. 



(7.) The mean of the two results will be the diflTerence in the 

 length of the standards, free from the effect of the curvature of the 

 ways along which the carriage moves. The same result will be 

 obtained if the microscope carriage is moved along the cylindrical 

 ways, the bars remaining stationary during each independent opera- 

 tion ; but the curvature eliminated will in this case be that which 

 belongs to the cylindrical ways. 



(c.) Comparison by means of Stops and one Microscope. 



The stops H H^ move freely upon the cylinders X X, but they are 

 capable of being clamped to them with great firmness by the levers 

 shown near L. At one end they terminate in an oval projection of 

 tempered steel, which is hidden behind the cylinder X, in Fig. 1. 



