PHYSICS. 353 



used as a mixture of methyl and ethyl chlorides boiling at about 2.5° 

 under normal atmospheaic pressure. The greatest daily change of tem- 

 perature observed in the room during a fourteen-day test was .04°, and 

 the least .01°. The details of construction and of management are given 

 in the paper referred to. (Nature, April, 1887, xxxiiii, 596. See also 

 Pernet, Nature, May, 1886, xxxiii, 48.) 



On thermometric questions Whipple has described the method in use 

 at the Kew Observatory for the comx^arison and verification of ther- 

 mometers at the freezing point of mercury. (Phil. Mag. January, 1886, 

 V, XXI, 27.) Pickering has published notes on the calibration and stand- 

 ardizing of mercurial thermometers, and has described a modification of 

 his own, in delicate thermometers for calorimetrical work. (Phil. Mag., 

 March, April, 1886, V, xxi, 180, 330.) 



2. Expansion and change of state. 



Weber has proposed the use of the pendulum method for determin- 

 ing the CO- efficient of expansion of solids. The time of oscillation of a 

 solid body in vacuo depends on the form of the body, on its mass, and 

 on the distance of its particles from the axis of rotation. At two differ- 

 ent temperatures these distances are different, and hence the times of 

 oscillation differ. In other words, there is for every body a definite 

 relation between its temperature %, its expansion coefficient a, its di- 

 mensions d, and its time of oscillation t, which is expressed by the for- 



mula a= —jn m' The time of oscillation is to be obtained by noting 



exactly the interval between two passages of the pendulum through the 

 vertical, six hours apart, using for this purpose a Hipj) chronoscope 

 whose hands are put in motion by the j)endulum of an accurate clock 

 and stopped by the pendulum under experiment. The temperature is 

 determined by means of a thermo-electrical coui^le. In this way the 

 author hopes for a precision of one hundred-thousandth part in the co- 

 efficient, in place of one six-thousandth with the present methods. (0. 

 E,, September, 1886, cm, 553.) In a subsequent note Guillaume criti- 

 cises this method, and shows that with the best possible installation a 

 precision of not over one three-hundredth part is to be expected by its 

 use ; and, moreover, the apparatus is complicated. (C. R., October, 

 1886, cm, 689.) 



A simple apparatus for showing that a wire is cooled when it is 

 stretched has been described by Dorn. A steel vertical wire, about 0.7 "'" 

 in diameter, is clamped at the upper end, while at the lower is fixed a 

 scale-pan in which weights can be placed. Eound two adjacent portions 

 of this wire pieces of fine German-silver wire and of steel wire are wound, 

 the ends of which are so connected with a galvanometer that they form 

 a thermoelement. When weights are placed in the pan the galvanom- 

 eter shows a cooling effect, and when, after a time, they are removed, a 

 warming effect. With a Wiedemann's galvanometer, almost dead-beat, 

 H. Mis. 600 23 



