2 1 S KT1 [ E K AM) G K A VITATK >NAL MATTER. 



acquired by a particle, when the disturbance consists in the superpo- 

 sition of different series of plane polarized waves: and we may con- 

 clude, for every kind of radiation of light or heat except a series of 

 homogeneous circularly polarized waxes, that the mechanical value of 

 the disturbance kept up in any space is less than the product of the 

 mass into the square of the greatest velocity acquired by a vibrating. 

 particle in the varying phases of its motion. How much Less in such 

 a complex radiation as that of sunlight and heat we can not tell. 

 because we do not know how much the velocity of a particle may 

 mount up. perhaps even to a considerable value in comparison with 

 the velocity of propagation, at some instant by the superposition of 

 different motions chancing to agree: hut we may he sure that the 

 product of the mass into the square of an ordinary maximum velocity, 

 or of the mean of a great many successive maximum velocities of a 

 vibrating particle, can not exceed in any great ratio the true mechan- 

 ical value of the disturbance. 



Sec. 'i. Recurring, however, to the definite expression for the 

 mechanical value of the disturbance in the case of homogeneous cir- 

 cularly polarized light, the only case in which the velocities of all 

 particles are constant and the same, we may define the mean velocity 

 of vibration in any case as such a velocity that the product of its square 

 into the mass of the vibrating particles is equal to the whole mechan- 

 ical value, in kinetic and potential energy, of the disturbance in a certain 

 space traversed by it: and from all we know of the mechanical theory 

 of undulations, it seems certain thai this velocity must he a very small 

 fraction of the velocity of propagation in the most intense light or radi- 

 ant heat which is propagated according to known laws. Denoting this 

 velocity for the case of sunlight at the earth's distance from the sun by 

 /•. and calling W the mass in grams of any volume of the Luminiferous 

 ether, we have the mechanical valueof the disturbance in the same space, 

 in terms of terrestrial gravitation units. 



\Y ., 



g 



where g is the number 981, measuring in (C.Gr.S.) absolute units of 



force, the force of gravity on a gram. Now. from Pouillet's obser- 



, , . ,, , , , . , ,. 1 ', L235X 46000 



vat ion. we found in the last footnote on section 1 above, 



for the mechanical value, in centimeter-grains, of a cubic centimeter 

 of sunlight in the neighborhood of the sun: and therefore the mass, in 

 grams, of a cubic centimeter of the ether, must he given by the equa- 

 tion, 



,„ 981X1235X46000 

 W= - v • 



