OP ARTS AND SCIENCES. 



301 



defined by the motiou of the microscope along the ways upon which 

 it rests. 



(3.) Me^lianism for the adjustment of the bars under the micro- 

 scopes in the shortest possible time, by a combined quick and slow 

 motion, which should not require any disturbance of the standards 

 after tlu'y are placed in position upon their neutral points of support. 

 The three quick and slow movements required are : 

 (a.) A motion of translation, longitudinally. 

 (b.) A motion at right angles to the motion of translation. 

 (c.) A vertical motion. 



(4.) Mechanism which will allow comparisons of line-measures 

 by as many independent methods as possible. 



(5.) Mechanism for the convenient comparison of line-measures 

 with end-measures. 



(6.) Mechanism which will allow an easy comparison of the 

 subdivisions of any unit, without regard to their magnitude. 



Figures 2 and 3 present perspective views of the apparatus, and 

 show the general relations of all the parts to each other. The plan 

 of the comparator is shown in Fig. 4. The front elevation is shown 

 in Fig. 1, in which the part above the line A B is lifted, in order that 

 it may be seen clear of the tables. The form of the micrometer of 

 the microscope is shown in Fig. 6. The general form of the micro- 

 scope carriage, with microscope in position, is shown in Fig. 7. An 

 end view of the carriage G is shown in Fig. 5. 



The first condition named is fulfilled by mounting the bed-plate 

 L L, Fig. 1, upon brick piers having granite cap-stones. The bed- 

 plate of the carriage G, which supports the tables S, rests upon inde- 

 pendent piers. Both of these sets of piers are disconnected from 

 the floor of the comparing-room. 



The second condition is fulfilled by the use of the hollow steel 

 cylinders X. The microscope plate, K, moves freely upon these 

 cylindrical ways. It can also be moved by a rack and free pinion. 

 The rack is behind the cylinder X, but the head, R, of the pinion is 

 shown. The flexure of the cylinders is taken up by the levers P, T, 

 which are not shown in their proper position. They should be nearer 

 the centre. The required pressure is supplied by weights attached to 

 cords fastened to the ends of the lever P. 



It is due to the excellent workmanship of the Pratt and Whitney 

 Company that these ways have no appreciable horizontal curvature, 

 while the very slight deflection due to flexure is readily taken up by 

 the flexure levers. An experience of nearly two years has shown that 



