136 ANNUAL EECORD OF SCIENCE AND INDUSTRY. 



purposes, lie devised the following construction : A glass tube 

 dips into a bulb of pure India rubber. The interior of the 

 bulb is filled with colored water, which, by pressure upon the 

 bulb, is forced up in the tube, the upper part of which is fill- 

 ed with air, which is thereby compressed. Attached to the 

 tube is a scale whose divisions represent pounds. As fifteen 

 pounds' pressure to the square inch is required to compress 

 the given body of air to one half its volume, of course that 

 amount of force brought to bear upon the bulb will press the 

 column of water half-way up the scale. This apparatus, be- 

 sides the advantage of simplicity, cheapness, and great accu- 

 racy, has the further convenience that the bulb receiving the 

 pressure is of a convenient shape, and can be used by persons 

 of large or small hands. Again, the bulb is adapted to receive 

 pressure exerted by all the flexors of the hands, and, finally, 

 the action of the muscles is the same at difierent times, the 

 same group of muscles being always brought into play, so that 

 correct comparative tests may be made from day to day. 

 PsycJiological and Medico-Legal Journ.^ April^ 1875, 256. 



ON THE ELASTICITY OF EOCK-SALT. 



Some physical experiments have been made by Pro- 

 fessor Groth, of Strasburg, upon the elasticity of rock-salt. 

 They involved, first, the preparation of rods of crystalline salt, 

 some three inches in length and one fiftieth of an inch in 

 thickness; these were made perpendicular to the plane of the 

 cube, and also to the dodecahedron. For the determination 

 of the coefiicient of elasticity the rod of salt was fastened to 

 a much longer rod of brass, and then set in vibration in the 

 usual way; sand strewed on the vibrating rods determined 

 the position of the nodes, and these in connection with the 

 thickness gave the elasticity of the salt relatively to the 

 brass. By comparing two rods, made as described, the effect 

 of the intervening brass rod was eliminated, and the coeffi- 

 cient of elasticity for the two determined. The ratio ob- 

 tained was 1 to 1^ (nearly), expressing the relation between 

 the elasticity normal to the dodecahedral planes with that 

 normal to the cubic planes. This result is important, as 

 showing that the elasticity in an isometric mineral, like rock- 

 salt, is not throughout uniform, but is dependent upon the 

 direction. 



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