THEORY OF LIGHT AND COLOURS. 87 



ation will naturally be ftraight before it ; behind it, the mo- 

 tion will be equal, in a contrary direclion ; and, at right an- 

 gles to the line of vibration, the undulation will be evanefcent. 

 Now, in order that fuch an undulation may continue its 

 progrefs to any confiderable diftance, there mult be in each 

 part of it, a tendency to preferve its own motion in aright 

 line from the centre ; for, if the excefs of force at any part 

 were communicated to the neighbouring particles, there can 

 be no reafon why it fhould not very foon be equalifed through- 

 out, or, in other words, become wholly extinct, fince the 

 motions in contrary directions would naturally deftroy each 

 other. The origin of found from the vibration of a chord is 

 evidently of this nature ; on the contrary, in a circular wave 

 of water, every part is at the fame inftant either elevated or 

 deprefTed. It may be difficult to (how mathematically, the 

 mode in which this inequality of force is preferved ; but the 

 inference from the matter of fact, appears to be unavoidable ; 

 and, while the fcience of hydrodynamics is fo imperfect that 

 we cannot even folve the fimple problem of the time required 

 to empty a veffel by a given aperture, it cannot be expected 

 that we fhould be able to account perfectly for fo complicated 

 a feries of phenomena, as thofe of elaftic fluids. The theory Theory of Huy- 

 of Huygens indeed explains the circumftance in a manner to- g€ns * 

 lerably fatisfactory : he fuppofes every particle of the medium 

 to propagate a diftinct undulation in all directions ; and that 

 the general effect is only perceptible where a portion of each 

 undulation confpires in direclion at the fame inftant ; and it is 

 eafy to fliovv that fuch a general undulation would in all cafes 

 proceed reel il in early, with proportionate force ; but, upon 

 this fuppofition, it feems to follow, that a greater quantity of 

 force muft be loft by the divergence of the partial undulations, 

 thari appears to be confiftent with the propagation of the ef- 

 fect to any confiderable diftance. Yet it is obvious, that 

 fome fuch limitation of the motion muft naturally be expected 

 to take place ; for, if the intenfity of the motion of any par- 

 ticular part, inftead of continuing to be propagated ftraight 

 forwards, were fuppofed to affect the intenfity of a neigh- 

 bouring part of the undulation, an impulfe muft then have 

 travelled from an internal to. an external circle in an oblique 

 direction, in the fame time as in the direction of the radius, 

 and confequently with a greater velocity ; againft the firft 



propofition. 



