C. GENERAL PHYSICS. 175 



while its motion is entirely under the control of the observer. 

 Professor Zollner's apparatus was designed expressly to 

 measure attractive forces and slight changes of level. Pro- 

 fessor Rood proposes to apply his own similar apparatus to 

 the study of minute changes in the dimensions of solid bod- 

 ies, for which purpose he gives it such dimensions as to im- 

 part to it an unprecedented delicacy. The main difficulty in 

 the use of this apparatus is the fact that it is exceedingly 

 sensitive to the most distant and unseen sources of disturb- 

 ance. Thus Professor Rood remarks that children, playing 

 at a distance of 360 feet, caused temporary deflections of one 

 or two scale divisions ; and similar deviations were caused 

 by the lower notes of an organ in a neighboring church, the 

 middle and higher notes producing no sensible results. 

 These effects upon the apparatus can be eliminated, howev- 

 er, by making a sufficient number of observations, the evils 

 caused by them being only temporary. As usual in all inves- 

 tigations, the effects of temperature are the most insidious. 



As illustrating the fineness of the measurements that can 

 be made with the horizontal pendulum, Professor Rood gives 

 some figures showing that the one eighteen-millionth part of 

 an inch becomes a sensible quantity ; whereas hitherto, with 

 the best optical and mechanical means, it has been hardly 

 possible to measure the two one-hundred-thousandth part of 

 an inch. Am. Jour. Sci., 1875, IX., 441. 



THE ELASTICITY OF BARS OF ICELAND SPAR. 



Dr. G. Baumgarten mentions that a lecture of Professor 

 Neumann on the theory of elasticity, in which he called at- 

 tention to the interest that would attach to the determina- 

 tion of the co-efficients of elasticity in crystalline bodies, led 

 him to undertake this labor, and that, so far as he knows, his 

 own are, as yet, the first direct observations on the elastic 

 properties of crystals. Voigt, however, has since then in- 

 vestigated the elasticity of the crystals of rock salt. Iceland 

 spar was chosen by Dr. Baumgarten, among other reasons 

 because, in reference to its physical and optical properties, 

 it is better known than almost any other mineral. His de- 

 terminations of its elasticity w^ere made by measuring the 

 bending of bars of spar, when pressed in various directions, 

 and which had been cut in different directions from the crys- 



