I. SPECIAL COLLECTIONS. 35 



lens, sliding on a horizontal plate parallel to the other plate. The measuring 

 field is about two centimetres in extent, or a little less than 1 inch. Value 

 of one division of each micrometer = '00003097 inch, or 0' 0007866 milli- 

 metres. 



172. Apparatus for determining the Expansion of 

 Standard Bars. Constructed by Troughton and Simms. 



The trough containing the steel bar with projecting points, distant 1 yard and 

 1 metre, is filled with melting ice to secure constant length at the temperature 

 of C. The standard bar is placed in the lower trough, with two standard 

 thermometers, and is raised gently against the points. Their impression shows 

 the constant length on the bar at its noted temperature in ordinary air. Next 

 fill the lower trough with melting ice, and take impressions to show the con- 

 stant length on the bar when at C. Then fill the lower trough with water, 

 and raise to boiling point, or other less high temperature, by the heat from 

 the gas jets underneath, and take impressions to show the constant length 

 on the bar when at 100 C., &c. From the difference of these lengths accu- 

 rately measured under micrometer microscopes, the rate of expansion of the 

 bar is deduced. 



173. Large Callipers for measuring diameter and depth of 

 cylindrical or other measures. Constructed by Troughton and 

 Simms. 



These are made on the same principle as the instruments used for measuring 

 shot and the bore of guns at Woolwich. They measure diameters up to 

 24 inches and within O'OOl inch by the aid of a vernier. 



174. Model of Sub-divided Yard with comparing appa- 

 ratus, for the use of local inspectors of weights and measures. 

 Constructed by Troughton and Simms. 



The tested yard measure is placed with its zero defining line immediately 

 under that of the standard. By running the eye-piece along the upper guide 

 bar, each defining line is accurately compared and differences determined to 

 less than 0' 01 inch by means of the small supplementary sub-divided inch 

 measure placed also under the eye-piece. This apparatus is described and 

 illustrated in Appendix III., 7th Annual Report of Warden of the Standards, 

 1873. 



175. Spherometer for measuring spherical curves, with true 

 gun-metal plane\ Used for measuring the flexure of the middle 

 of the glass disc placed upon the Imperial Standard bushel. Con- 

 structed by Troughton and Simms. 



When the horizontal plane is made to rest with its three triangular flattened 

 points upon the true plane, the central screw with its micrometer head is 

 accurately adjusted in the same plane, and its reading noted. By substituting 

 for the plane the surface to be tested, its convexity or concavity is determined 

 from the difference of the reading of the micrometer, either or + . Value 

 of one revolution of the screw = 0' 01 inch, and of one division of the mi- 

 crometer=0'0001 inch. By interposing a bright beam of light between the 

 point of the screw and the surface tested, and by estimation of 1 division, 

 accurate measurements have been made to 0' 00001 inch. This instrument 

 is described in Appendix X., 6th Annual Report of Warden of the Standards, 

 1872. 



C i' 



