H 
Dr. Young’s Lecture on 
the centre ; for, if the excess of force at any part were commu- 
nicated to the neighbouring particles, there can be no reason 
why it should not very soon be equalised throughout, or, in 
other words, become wholly extinct, since the motions in con- 
trary directions would naturally destroy each other. The 
origin of sound 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 same instant either elevated or depressed. It may 
be difficult to show mathematically, the mode in which this 
inequality of force is preserved ; but the inference from the 
matter of fact, appears to be unavoidable ; and, while the science 
of hydrodynamics is so imperfect that we cannot even solve the 
simple problem of the time required to empty a vessel by a 
given aperture, it cannot be expected that we should be able to 
account perfectly for so complicated a series of phenomena, as 
those of elastic fluids. The theory of Huygens indeed explains 
the circumstance in a manner tolerably satisfactory : he sup- 
poses every particle of the medium to propagate a distinct un- 
dulation in all directions ; and that the general effect is only 
perceptible where a portion of each undulation conspires in 
direction at the same instant ; and it is easy to show that such a 
general undulation would in all cases proceed rectilinearly, with 
proportionate force; but, upon this supposition, it seems to 
follow, that a greater quantity of force must be lost by the 
divergence of the partial undulations, than appears to be con- 
sistent with the propagation of the effect to any considerable 
distance. Yet it is obvious, that some such limitation of the 
motion must naturally be expected to take place ; for, if the 
intensity of the motion of any particular part, instead of conti- 
nuing to be propagated straight forwards, were supposed to 
affect the intensity of a neighbouring part of the undulation, an 
