278 SURVEY OF THE COAST 



English base apparatus, where this motion was impeded by 

 the weight of the whole box and its apjjaratus. 



Plate IV, fig. 1^2, presents a horizontal and vertical sec- 

 tion of the whole apparatus, placed as when in use. 



The whole bar between the two foci of the microscopes 

 consists of an assemblage of four iron bars each of two me- 

 tres in length, and exactly of the same breadth and thickness 

 as the metres constructed and standarded by the Commit- 

 tee of Weights and Measures in Paris in i799. Tliey are 

 joined together end to end by means of two iron clamping 

 pieces AA, a section of which, perpendicular to the bars, is 

 seen, in one-fourth of its real size, in Plate I. fig, 2. Each 

 of these pieces being clamped to its bar I)y the screw B, the 

 two corresponding pieces of the adjoining bars are screwed, 

 in order to make the contact of the bars, by the longitudinal 

 screws CC, above and below the bars. The bars can be ea- 

 sily brought in contact, as any gap would be immediately 

 observed ; and there is no fear of compression, because the 

 instant of contact is easily ascertained, and before any com- 

 pression of the metal could take place, the friction of the 

 screws B would give way, and restore equilibrium and con- 

 tact. 



This assemblage of bars stands edgeways upon rollers F 

 of one-third of an inch in diameter, placed at short distances 

 (so that each double metre bar may have four) in brass 

 pieces DD, which bear also pillars of about one inch in 

 length EE, rising on both sides of the bars and presenting a 

 rounded surface to them like a section of a cylinder. Tliis 

 surface is near enough to the bars to prevent their vacillation, 

 but not their motion. Different sides of tliem are seen at 

 Plate HI. fig. 2, 4, n, and a plan of one of them is seen at 

 fig, 3. In one of these pillars, which is directly above the 

 screws for the motions of the bar, there is a clamping screw 

 G. (Plate I. fig. 4) by which the longitudinal motion of the 

 bars on these supporting pieces and rollers is arrested. From 

 this point, therefore, the expansion of the bars is allowed to 

 act with full freedom upon the rollers F. 



