Waves generated by Impact. 99 



to obtain a quantitative measurement of the relative inten- 

 sities in actual experiment. It was then noticed that the 

 intensity practically vanishes on a cone making an angle of 

 about 67° with the line joining the centres. 



After many trials an apparatus has been devised which 

 appears to satisfy the necessary conditions of extreme sensi- 

 tiveness, suitability for quantitative work, and convenience in 

 actual use. This apparatus, which is believed to be of a new 

 type, is based on a ballistic principle. Its construction is 

 quite distinct from that of the phonoscope invented by 

 Dr. Erskine-Murray, or other similar devices in which the 

 motion of a membrane or disk on which the sound-waves 

 are incident deflects a pivoted mirror connected with it. 

 As a matter of fact, a phonoscope of the ordinary type was 

 given the first trial, but proved quite unsuitable for the 

 present work, as the deflexion observed with it was too small 

 and too sudden to be capable of measurement by visual obser- 

 vation, or even for satisfactory photographic registration. 



The apparatus finally devised and employed consists of a 

 small mirror attached to a pivoted axle whose free move- 

 ment is controlled by a fine spiral spring. (In practice the 

 balance-wheel and hair-spring of a watch proved very 

 satisfactory, the mirror being attached radially to the wheel 

 with a little cement.) The sound is received by a horn over 

 the tubular end of w T hich a mica disk is fixed. A sharp 

 metal pointer is fixed normally to the centre of the disk and 

 its end lightly touches the pivoted mirror referred to above, 

 but is not connected with it. The light from a slit illumi- 

 nated by an arc-lamp is condensed by a lens on the pivoted 

 mirror, the reflected light forming a sharp image of the slit 

 on a distaut graduated screen. For the production of impact, 

 the balls are hung side by side by bifilar suspension from a 

 framework which is capable of rotation round a vertical axis. 

 The balls can be made to impinge on each other in a direction 

 making any desired angle with the axis of the horn by simply 

 rotating the framework. This angle can be read off on a 

 graduated circle fixed below the frame. In order to obtain 

 perfect regularity in the sound of the impact and to avoid 

 unnecessary reflexions from closely contiguous bodies, an 

 electromagnetic arrangement was used by which the balls 

 could be automatically dropped on breaking the circuit. 



As soon as the balls collide, the souud-wave generated by 

 the impact passes through the horn and impinges on the 

 mica disk. The motion of the pointer attached to the disk 

 gives a kick to the pivoted mirror, which moves off freely 

 until it is brought to rest by the controlling spiral spring. 



H2 



