On Porous Bodies in relation to Sound. 181 
tively connected, and thereby submitted to the process of 
electric distribution. 
To the theory of the sun's electric potential might still be 
opposed the objection that the electric attraction between the 
sun and the planets, and the repulsion which the latter would 
necessarily exert upon one another and upon their satellites, 
would modify the basis of the astronomical calculations, since 
then, besides gravitation, an additional force, the electrical, 
would have to be taken into account. 
This objection is perfectly legitimate. But as electric force, 
equally with gravitation, stands in the ratio of the square 
of the distance of the centres, the paths of the planets would 
remain unaltered if a part of the gravitational were replaced 
by an electrical attraction. Only the calculated ratio of the 
masses of the sun and planets to that of the earth would be 
changed. These alterations would be sensible, especially in 
the case of the small planets and the satellites, since electric 
force is a function of the surface. On the other hand, how- 
ever, the disturbing influences exerted by the planets and their 
satellites upon one another's paths must be changed if gravi- 
tation be diminished by electric repulsion. 
Perhaps it is reserved for astronomy to bring out from the 
perturbations of the paths of Mercury, the asteroids, and the 
satellites the demonstration of the existence or nonexistence 
of an electric potential of the sun. 
XXYI. On Porous Bodies in relation to Sound. By Lord 
Ratleigh, JD.C.L., F.R.S., Cavendish Professor of Physics 
in the University of Cambridge* '. 
IN Acoustics we have sometimes to consider the incidence 
of aerial waves upon porous bodies, in whose interstices 
some sort of aerial continuity is preserved. Tyndall has shown 
that in many cases sound penetrates such bodies, e. g. thick 
pieces of felt, more freely than would have been expected, 
though it is reflected from quite thin layers of continuous 
solid matter. On the other hand, a hay-stack seems to form 
a very perfect obstacle. It is probable that porous walls give 
a diminished reflection, so that within a building so bounded 
resonance is less prolonged than would otherwise be the case. 
When we inquire into the matter mechanically, it is evi- 
dent that sound is not destroyed by obstacles as such. In the 
absence of dissipative forces, what is not transmitted must 
be reflected. Destruction depends upon viscosity and upon 
* Communicated by the Author. 
