216 ETHER AND GRAVITATIONAL MATTER. 



Joule's mechanical equivalent of the thermal unit. Thus the value of 

 solar radiation per second per square centimeter at the earth's distance 

 from the sun. estimated at 1,235 cm. -grams, is the same as the mechan- 

 ical value of sunlight in the luminiferous medium through a space of 

 as many cubic centimeters as the number of linear centimeters of prop- 

 agation of light per second. Hence the mechanical value of the whole 

 energy, kinetic and potential, of the disturbance kept up in the space 

 of a cubic centimeter at the earth's distance from the sun a is 



1235 1-12 , 



3xlo io ,or JqTo ot a em. -gram. 



Sec. 2. The mechanical value of a cubic kilometer of sunlight is 

 consequently 412 meter-kilograms, equivalent to the work of one 

 horsepower for live and four-tenths seconds. This result may give 

 some idea of the actual amount of mechanical energy of the luminif- 

 erous motions and forces within our own atmosphere. Merely to 

 commence the illumination of 11 cubic kilometers requires an amount 

 of work equal to that of a horsepower for a minute; the same amount 

 of energy exists in that space as long as light continues to traverse 

 it, and, if the source of light be suddenly stopped, must pass from it 

 before the illumination ceases.' 1 The matter which possesses this 

 energy is the luminiferous medium. If, then, we knew the velocities 

 of the vibratory motions, we might ascertain the density of the 

 luminiferous medium; or, conversely, if we knew the density of the 

 medium, we might determine the average velocity of the moving 

 particles. 



Sec. 3. Without any such definite knowledge we may assign a 

 superior limit to the velocities and deduce an inferior limit to the quan- 

 tity of matter by considering the nature of the motions which con- 

 stitute waves of light. For it appears certain that the amplitudes of 

 the vibrations constituting radiant heat and light must be but small 

 fractions of the wave lengths, and that the greatest velocities of the 

 vibrating particles must be very small in comparison with the velocity 

 of propagation of the waves. 



Sec. 4. Let us consider, for instance, homogeneous plane polarized 

 light, and let the greatest velocity of vibration be denoted by v\ the 

 distance to which a particle vibrates on each side of its position of 



a The mechanical value of sunlight in any snare near the sun's surface must be 

 greater than in an equal space at the earth's distance in the ratio of the square of the 

 earth's distance to the square of the sun's radius — that is, in the ratio of 46,000 to 1 

 nearly. The .mechanical value of a cubic centimeter of sunlight near the sun must, 

 therefore, he 



] - :] '] ' "''J'"", or about .0010 of acm.-gram. 



''Similarly we find 4,140 horsepower for a minute as the amount of work required 

 to generate the energy existing in a cubic kilometer of light near the sun. 



