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 their length is, as will be seen hereafter, exactly quadruple; the rapidity of their propagation depends, as in air, on the relation between the elastic force of the fluid and its density.

When a sonorous wave excited in air arrives at the surface of a solid body, its impact produces in the parts of that body a motion, insensible indeed, but nevertheless real, which sends it back. If the body, instead of being solid, is of a gaseous form, the reflexion takes place equally, but there is produced in the gas a sensible undulation depending on the impression that its surface has received. Luminous undulations ought to produce a similar effect when the medium in which they are excited is terminated by a body in which the density of the ethereal fluid is different; that is to say, there must be produced a reflected wave and one transmitted; which is, in fact, what we call reflexion and refraction. In this system, the intensities of rays of light must be measured by the vis viva of the fluid in motion, that is, by the product of the density of the fluid by the square of the proper velocity of its particles.

To confirm these analogies, already very remarkable, it would be necessary to follow up their consequences further with calculations; but unfortunately this cannot be done rigorously. The subject of undulations thus sent back or transmitted in oblique motions, is beyond the existing powers of analysis. In the case of perpendicular incidence the phænomenon becomes accessible, but then it teaches nothing as to the general direction of the motion communicated, as the propagation must be continued in a straight line, if for no other reason, on account of there being no cause why it should deviate from it; nevertheless in this case theory indicates the proportions of intensity for the incident and reflected waves, which appear, in fact, tolerably conformable to experiments on light, which is, at any rate, a verification as far as it goes.

When the ear receives at once two regular and sustained sounds, it distinguishes, besides those sounds, certain epochs at