OF ARTS AND SCIENCES. 



119 



It is nearly in line with the centre of gravity of K, which has a 

 corresponding flat surface of tempered steel at each end. 



These stops receive a slow motion by means of a screw, which is 

 shown near the pillow blocks in Fig. 3. 



(1.) The stops H H^ are set approximately, e. g. 1 meter apart plus 

 the length of K, and are securely clamped. 



(2.) The plate K is brought into contact with the left-hand stop. 



(3.) The standard to be compared is placed upon S, and the car- 

 riage is moved by the arm D till the left-hand defining line is adjusted 

 under M. 



(4.) The plate K is then brought into contact with the right-hand 

 stop, and the other defining line is brought into position and into 

 focus under M by the adjustments already described. 



(5.) These adjustments having been completed, K is brought into 

 contact, first with H, and then with H^, and the micrometer is read 

 for each contact. 



(G.) These operations are rei^eated for each standard comi^ared. 

 Since each one has been compared with the invariable distance be- 

 tween the stops, the data are thereby furnished for the comparison 

 with each other. In practice, the standard with which other stan- 

 dards are to be compared is always compared first with the distance 

 between the stops, the record being written at the left of the page. 

 The following example will illustrate the form of record. R signi- 

 fies contact with the right sto}), and L with the left stop. 



Under these definitions, the second standard is longer than the first, 

 when {R' — U) — (i? — L) is positive. In this case, therefore, 

 C. S. is 1.8 div. longer than T. 



With the microscope of Comparator No. 1, however, this order is 

 reversed. In that case, the second standard is shorter than the first, 

 when {R' — L') — (R — L) is positive. 



(d.) Comparison hy means of Stops, and with two Microscopes. 



(1.) The standards to be compared are placed side by side upon 

 the table S, and the carriage is placed at a convenient point upon E. 



