260 Prof. W. Stroud. [June 19, 



pervading molecular effect. This conception fully explains our 

 results of two years ago that bars of ice with the axis transverse 

 yield neither to pull nor thrust. If we had tried a bar with the optic 

 axis oblique, it would have stretched readily enough. 



The rate of distortion was very irregular, showing a strong ten- 

 dency to increase with the length of time for which the weight was 

 applied. When extra weight was put on, the rate increased more 

 than in proportion to the weight itself, but less than in proportion to 

 its square. The effect of temperature was generally masked by the'se 

 others, but there could be no doubt of its existence ; the rate at 2 

 being in one case twice or three times as great as cceteris paribus at 

 -10. 



Plasticity, due to sliding planes (Gleitflachen), has been shown to 

 exist in rock salt, Iceland spar, &c., by Reusch and others. In rock 

 salt these planes are parallel to the faces of the rhombic dodecahedron, 

 and in general there are several different sets. As long ago as 1867, 

 Reusch suggested their existence in ice as a means of explaining. the 

 observed plasticity. I find that the observation that an ice crystal is 

 plastic was made by Hagenbach in 1881, but he did not further 

 investigate the matter. 



VII. te Preliminary Note on a New Magnetometer." By W. 

 STROUD, D.Sc., Professor of Physics, Yorkshire College, 

 Leeds. Communicated by Professor A. W. RfJCKER, F.R.S. 

 Received May 30, 1890. 



The determination of the horizontal component of the earth's mag- 

 netic field is of great importance, not only for the purpose of magnetic 

 surveys, but also for the determination of the absolute strength of an 

 electrical current, a measurement frequently required, not only for 

 scientific work of various kinds, but also for the calibration of 

 ammeters and voltmeters, and other electrical measuring instruments. 



The usual method of measuring this important quantity is that of 

 Gauss, but the method is so long and laborious, and the apparatus 

 requisite for accurate determination so expensive, that the measure- 

 ment of H is avoided whenever practicable. The writer, having 

 devised an instrument capable of determining H with great expedition 

 and accuracy, ventures to think that a description of the instrument 

 may not be without interest. 



Gauss's method consists, as is well known, in finding (1) the deflec- 

 tion produced v^on a small magnetic needle by a large magnet of 

 moment M, placed at a known distance from, and in a certain position 

 with regard to, the needle; and (2) the time of vibration of the 

 deflecting magnet when suspended so as to oscillate in a horizontal 



