114 Astronomical and Nautical Collections, 



by each of the forces, is raised or depressed with a velocity 

 equal to the sum of the effects of the two separate impulses, 

 or to the double of either of them taken singly, since they 

 are now supposed to be equal. Between these points of per- 

 fect agreement and complete opposition, which exhibit, one 

 the total absence of motion, the other the maximum of oscil- 

 lation, there are an infinity of intermediate points, at which 

 the alternate motion takes place with more or less of energy, 

 accordingly as they approach more or less to the places of 

 perfect agreement, or of complete opposition of the two 

 systems of motion which are thus combined, or superinduced 

 on each other. 



The waves which are propagated in the interior of an 

 elastic fluid, though very different in their nature from those 

 of a liquid like water, produce mechanical effects by their 

 interference, which are exactly of the same kind, since they 

 consist in alternate oscillatory motions of the particles of the 

 fluid. In ^fact,.it is sufficient that these motions should be 

 oscillatoiy, that is, that the particles should be carried 

 by them alternately in opposite directions, in order that 

 the effects of one series of waves may be destroyed by those 

 of another series of equal intensity ; for, provided that the 

 difference of the route of the two groups of waves [derived 

 from the same origin] be such, that for each point of the 

 fluid the motions in one direction, belonging to the first series, 

 correspond to the motions, belonging to the second, in the 

 opposite direction, they must perfectly neutralise each other, 

 if their intensity is equal : and the particles of the fluid must 

 remain in repose. This result will always hold good, what- 

 ever may happen to be the direction of the oscillatory mo- 

 tion, with regard to that in which the undulations are propa- 

 gated ; provided that the direction of the oscillatory motion 

 be the same in the two series to be combined. In the waves 

 which are formed on the surface of a liquid, for example, the 

 direction of the oscillation is [principally] vertical, while the 

 waves are propagated horizontally, and consequently in a di- 

 rection perpendicular to the former ; in the undulations of 

 sound, on the contrary, the oscillatory motion is parallel to 

 the direction of the propagation of the sound, [or rather is 



