122 Astronomical and Nautical Collections. 



sand miles in a second, we do not mean, according to the 

 undulatory system, that such is the absolute velocity of the 

 ethereal particles ; but that the motion communicated to 

 the ether employs only a second to pass to a stratum at the 

 distance of 200 thousand miles from its origin. 



In proportion as the undulation becomes more distant from 

 the centre of agitation, the motion, spreading over a greater 

 distance, must be weakened in every part of the wave. It 

 is shown by calculation, that the amplitude of the oscillatory 

 motion, or the absolute velocity of the particles concerned in 

 it, is inversely proportional to the distance from the centre 

 of agitation. Consequently, the square of this velocity is 

 inversely proportional to the square of the distance, and the 

 intensity of the light must be inversely as the square of the 

 distance from the luminous point. It must be remarked, that, 

 for the same reasons, the sum of the living forces of the whole 

 undulation remains unaltered ; for, on one side the length of 

 the undulation d, which may also be called its thickness, is 

 invariable, and its extent of surface augmenting in propor- 

 tion to the square of the distance from the centre, the quan- 

 tity, or mass of the fluid agitated, is proportional to the same 

 square : and since the squares of the absolute velocities are 

 diminished in the same proportion as the masses have aug- 

 mented, it follows that the sum of the products of the masses 

 by the squares of the velocities, that is to say, the sum of the 

 living forces, remains unaltered. It is a general principle of 

 the motion of elastic fluids, that however the motion may be 

 extended or subdivided, the total sum of the living forces re- 

 mains constant ; and this is the principal reason why the living 

 force must be considered as the measure of light, of which the 

 total quantity always remains very nearly the same, at least 

 as long as it continues to pass through perfectly transparent 

 mediums. 



It may be remarked, that black substances, and even the 

 most brilliant metallic surfaces, by no means reflect the whole 

 of the light which falls on them ; bodies which are imper- 

 fectly transparent, and even the most transparent, when of 

 great thickness, absorb also, to use a common expression, a 

 considerable portion of the light that is passing through 



