22 
Dr. Young's Lecture on 
in some degree from any that have been proposed by former 
authors, and is diametrically opposite to that of Newton ; but, 
both being in themselves equally probable, the opposition is 
merely accidental; and it is only to be inquired which is the 
best capable of explaining the phenomena. Other suppositions 
might perhaps be substituted for this, and therefore I do not 
consider it as fundamental, yet it appears to be the simplest and 
best of any that have occurred to me. 
PROPOSITION I. 
All Impulses are propagated in a homogeneous elastic Medium 
with an equable Velocity. 
, Every experiment relative to sound coincides with the obser- 
vation already quoted from Newton, that all undulations are 
propagated through the air with equal velocity; and this is 
further confirmed by calculations. (Lagrange. Misc. Taur. 
Vol. I. p. 91. Also, much more concisely, in my Syllabus of a 
course of Lectures on Natural and Experimental Philosophy, 
about to be published. Article 289. ) If the impulse be so great 
as materially to disturb the density of the medium, it will be no 
longer homogeneous ; but, as far as concerns our senses, the 
quantity of motion may be considered as infinitely small. It is 
surprising that Euler, although aware of the matter of fact, 
should still have maintained, that the more frequent undulations 
are more rapidly propagated. (Theor. mul and Conject. phys.) 
It is possible, that the actual velocity of the particles of the 
luminiferous ether may bear a much less proportion to the velo- 
city of the undulations than in sound ; for light may be excited 
by the motion of a body moving at the rate of only one mile 
in the time that light moves a hundred millions. 
