286 Mr. Challis on the Tlieort/ of the 



opposite directions along the cylindrical tube. There must be 

 a point at which the velocities are the same and in the same 

 order, in virtue of the two jn-opagations, but in opposite direc- 

 tions. At this point therefore the i)articles will be at rest. The 

 motion will in no respect be changed, if an indefinitely thin . 

 ricrid partition be placed at right angles to the axis of the tube, 

 just where the particles are stationary. The fluid will be divided 

 into two separate columns, in eacli of which the motions will 

 be the same as before. But plainly, the particles in one column 

 cannot be aftected by a pro[>agation which has its source in a 

 disturbance made in the other. Hence, the eflect of such dis- 

 turbance is supplied by rejection at the partition. It thus 

 appears that the obstacle gives rise to a series of reflected waves 

 exactly like the incident waves, and that the particles in contact 

 with the reflecting body do not move. 



7. I come now to a more particular consideration of the 

 discontinuity of tlie motion, and for the sake of illustration shall 

 begin with establishing a possible mode of generating the waves, 

 which are all along supposed to be of the primary type. Con- 

 ceive a .series of these waves to be propagated along the tube, 

 and at a certain point an indefinitely thin diaphragm the weight 

 of which is insensible, to be jilaced, and to be capable of moving 

 with perfect freedom in the direction of the axis. The motion 

 of propagation will evidently proceed just as if the diaphragnt 

 did not exist, since it only acts as a means of transmitting the 

 pressure of the particles on one side to those on the other. But 

 as the particles on one side do not communicate with those on 

 the other, the motion on the side looking in the direction of 

 the propagation, will remain the same as before, if the diaphragm, 

 by an independent cause, be made to oscillate just as it does 

 by reason of the propagation. It will thus become a disturbing 

 cause proper for generating a series of primary waves. By parity 



