PHYSICS. 597 



tubes, directed all alike, they rotated just as fast in just the same man- 

 ner as the others. When, however, one approached a node, the rota- 

 tion became slower, ceased, assumed the contrary direction in order 

 after further progress, to pause again, and next pass into the former 

 lively rotation. (Nature, November, 1885, xxxiii, 95.) 



Elsass has suggested an improved form of monochord in which the 

 vibrations of the string are produced by means of a siren. A small 

 disk is fixed somewhat eccentrically upon the axis of an ordinary siren, 

 .and to the uprights carrying the wheel work is attached, by means of 

 two screws, a support for a little bent lever, the conically-pointed axis 

 of which rests in two steel screws passing through the support. This 

 lever is capable of motion in a vertical plane, and presses with its 

 shorter and vertical arm against the eccentric disk while the longer and 

 Tiorizontal arm carries the end of a stretched thread at right angles to 

 the axis of the lever. When the siren is at rest the tension of the thread 

 retains the lever in position, and when it is sounding, and the eccentric 

 disk gives it a slight periodic motion, this tension brings it back. At 

 its other extremity the thread passes over a friction i)ulley, adjustable 

 in height, and is stretched suitably by weights placed in an attached 

 scale pan. When properly regulated and the siren put in motion, the 

 vibrations of the thread may easily be followed by the eye ; but as the 

 velocity increases, the friction of the disk against the lever produces a 

 noise; this suddenly ceases and the thread is seen to be in stationary 

 vibration of the fundamental form. The amplitude is considerable, 

 being often of three fingers' breadth for threads of a meter in length. 

 Moreover, the thread acts as a regulator for the sireu, and it is easy to 

 maintain its-pitch constant. Since the thread may be made to assume 

 different forms of vibration with a constant tension by varying the 

 speed of the siren, it differs from Melde's apparatus, where the rate of 

 vibration is constant and the tension varies. The author shows how it 

 may be used to illustrate Mersenne's laws. {Phil. Mag., January, 1885, 

 V, XIX, 48.) 



HEAT. 



1. Production of Heat. — Thermometry. 



Mallard and Le Chatelier have given a r6sum6 of their later re- 

 searches undertaken under the auspices of the Fire-damp Commission, 

 in order to determine, for a number a gaseous mixtures, the tempera- 

 ture of inflammation, the speed of propagation of this inflammation, and 

 the temperature given by the combustion in close vessels. They find 

 that inflammation of gaseous mixtures may be propagated in two dis- 

 tinct ways: either with a slow and uniform speed corresponding to the 

 deflagration of explosive solids, or with a speed extremely great cor- 

 responding to the explosion of the same bodies. The only difference 

 which exists between solid and gaseous explosives is that in the former 



