SaUARES, LEVELS, ETC. 177 



OPERA GLASS CASES. 



Caoutchouc ivory is a suitable material for the cases of opera 

 glasses, because of its lightness, and not being affected by change 

 of temperature. See plate , figs. 



QUADRANTS. 



Those parts of the frames of qjiadrants which have heretofore 

 been made of wood, may be made stronger and less liable to 

 warp and crack, from caoutchouc ivory and whalebone. See 

 plate , figs. 



BAROMETERS AND CHRONOMETERS. 



The cases of barometers and chronometers may be advan- 

 tageously moulded from caoutchouc whalebone. The cases of 

 clocks and time-pieces are also made of the same materials, in 

 the same way ; or when made of wood, they are veneered with 

 caoutchouc veneer. An improvement is also made in these 

 cases by packing them with gum-elastic sponge, so as to make 

 them quite water and air-tight, thereby excluding dust and 

 dampness. See plate , fig. 



WATER LEVELS. 



Caoutchouc ivory and whalebone are materials suitable for 

 water or spirit levels, on account of their solidity, and not being 

 liable to warp or crack. 



SQUARES AND BEVELS, CURVES, TRIANGLES, AND PARALLEL 



RULES. 



Caoutchouc whalebone is equally well adapted for these ar- 

 ticles, as all of them require tough, hard, and durable materials. 

 As they are all of them made in the same way in moulds, or 

 between plates of metal, they are described under one head. 



j(cj^a e^SSJo 



