44 SEC. 3. MEASUREMENT. 



176. Cathetometer for vertical measurements. Constructed 

 by Troughton and Simms. 



For example, for accurately reading a barometer or manometer: place 

 the cathetometer at a convenient distance, and adjust the cross wire of the 

 upper telescope to the level of the mercury in the glass tube, and that of the 

 lower telescope to the level of the mercury in the reservoir. The difference 

 of the two readings on the graduated scale of 42 inches gives the length of 

 the column of mercury to 0' 001 inch, by aid of the vernier. 



177. Stereometer for ascertaining the density of bodies by 

 determining their volume. Constructed by Troughton and Simms. 



This instrument was invented by M. Say ( Annales de Chimie,t. xxiii. p. 1 ,1 797), 

 for determining the specific gravity of gunpowder, and was used with some 

 improvements by Professor W. H. Miller (See Phil. Trans. 1856, part hi. p. 800.) 

 for determining the density of the platinum Kilogramme des Archives, during 

 his work of restoring the imperial standard pound. The solid body tested is 

 placed in the receiver communicating with the upper end of a vertical glass 

 tube, the lower end of which communicates with that of a second glass tube 

 having its upper end open to the air. The body should nearly fill the receiver, 

 which is screwed up air-tight in its place. Mercury is poured into the second 

 tube, and can be discharged by a stopcock at its lower end. Differences in 

 the relative height of the mercury in the two tubes are noted by means of the 

 cathetometer, as indicating the volume of compressed air under the two condi- 

 tions, when the body is in the receiver and when it is removed. The volume 

 of the body is deduced from the volume, of the mercury contained in the 

 tube between the different heights noted. 



178. Balance of new construction oscillating with steel 

 springs. This has been recently constructed by Oertling from a 

 design of Mr. Ar tings tall. 



Its principle is, that, instead of the beam and pans being suspended on 

 knife-edges, thin elastic steel springs are used, and adjustments of knife 

 edges from time to time are thus avoided. It is similar in construction to 

 Steinheil's silk ribbon balance. Its advantages as a balance for weighings, 

 where extreme scientific accuracy is not required, consist in its simplicity and 

 durability ; but it appears to be wanting in the sensibility and stability re- 

 quisite for a balance of precision. 



179. Model Kit of Apparatus for Local Inspectors of 

 Weights and Measures. Constructed by Oertling. 



This portable collection of all the necessary apparatus for comparing 

 imperial weights and measures has been taken from the Necessaire des 

 Verificaleurs, employed in France for verifying metric weights and measures, 

 with a view to its adoption in this country. It includes a Septimal Balance 

 by means of which a weight of 56 Ibs. is compared against 8 Ibs., the sum of 

 the standard weights contained in the kit. 



ISO. Experimental Gasholder for determining the internal 

 temperature. Constructed by Messrs. Wright & Co. 



By raising and lowering the bulb of the thermometers, the tubes of which 

 are made to slide through the top of the gasholder, the temperature of the 

 gas or air at various heights inside the bell can be read off through the glass 

 side, and the mean temperature determined. 



