478 



Prof. H. B. Dixon. On tjie Movements [June 5, 



Part IV. — On the Collision of Two Detonation-waves, and the Effect of 



Junctions in the Tubes. 



[In conjunction with K. H. Jones and J. Bower.] 



To study the effect produced in the collision of two waves of detona- 

 tion, the explosion tube (of lead pipe) was bifurcated into two arms of 

 equal length which were bent round and held the two ends of a strong 

 glass tube, in the centre of which the two waves met. The joint 

 between the lead and glass was made tight with india-rubber. 



The photographs obtained with this apparatus were puzzling. 

 Some of the " rebound waves" after the collision were much brighter 

 and travelled (backwards) much faster than the detonation-waves 

 themselves. Moreover, many photographs showed that the flame of 

 the explosion had been affected by some impulses causing a sudden 

 increase in its brightness and velocity, and producing a backward wave 

 (analogous to a reflected wave). 



The explanation of these appearances that first occurred to us was 

 that the flame was preceded by invisible sound-waves, travelling more 

 quickly than the flame in its initial phases ; that these sound-waves 

 became visible as soon as they met the flame moving towards them in 

 the opposite direction (as in our previous experiments on sound-waves), 

 and that, on the other hand, the visible flame meeting the sound-wave 

 was affected by the sudden increase of pressure, and continued its 

 journey with greater speed and luminosity. This explanation was at 

 once destroyed when we found similar impulses in a flame which was 

 sent through the apparatus in one direction only. 



It next occurred to us that these impulses might be due to the 

 explosion catching ujj its oivn sound-waves. If sound-waves are pro- 

 pagated through the gas from the point of ignition, the flame might 

 lap- behind the sound-waves at first and catch them after a run more or 



o 



less prolonged. The sound-waves when overtaken might cause 

 reflected sound-waves (made visible in the luminous gases), and the 

 explosion itself might become more intense owing to the collision. 

 Many experiments were undertaken to verify or disprove this 

 hypothesis ; but finally it was found that the explosion was affected as 

 it passed through the junctions between the lead and the glass, and 

 the "impulses " recorded in our photographs were due to the detona- 

 tion-wave, damped down at the junction, being regenerated by fits and 

 starts. 



The flame is not retarded by turning round a corner, even when 

 that is a sharp angle, nor is it damped down when the connecting 

 junction is made of stout flexible rubber. After many trials we found 

 that the only thing which mattered was the rigidity with which the 

 glass and metal were connected together. Any packing (such as 



