i88 3 J 
507 
The Velocity of Light . 
vibration. All sound carried to the centre of the earth is 
neutralised by a counter-vibration from the opposite side. 
The compressibility of ocean water at its mean temper- 
ature is 1-7-12*08 (the proportion of the polar zones to the 
sphere) less than that of water at 20° C. (the mean temper- 
ature of the surface layer of the sea), or only— 
0*0000417 = 1 23730 
of the depth of the ocean, which is also the proportion of 
the radius of the earth to the mean solar distance. The 
compressibility of the ocean by the atmosphere stands thus 
in a simple relation to the elliptic and polar motions of the 
earth, and to the elastic reactions of land, shell, and inter- 
venient stratum, in an opposite sense to those atmospheric 
changes for which the sea originally furnishes the material. 
When sound is collected and levelled by the tympane of 
the photophone, and entrusted to beams of light and radiant 
heat, centralised by a lens, the air-waves produced by our 
breath are transferred from the general atmosphere to the 
atmospheres and bodies of molecules, and re-transferred to 
the receiver again magnified from molecular to terrestrian 
dimensions. 
When, in the telephone, eledtric and magnetic motions 
are substituted for plain light and heat motions of the photo- 
phone, the difference is one of inter- and intra-molecular 
motions. The again enlarged sound-waves are delivered to 
and re-centralised and insulated by the ear-tube, which 
transmits the message of the outer world to the molecular 
eledtric conduction of the acoustic nerve for conveyance to 
the brain,— the camera where we store and arrange, to the 
degree of its capacity, all phonographs and photographs by 
which we refradt, refledt and concentrate, diffuse and absorb 
the intelligence of the infinite. 
“ La difference qui existe entre les sons graves et les sons 
aigus est si frappante pour nos organes qu’elle doit certaine- 
ment correspondre a quelque modification physique bien 
caradteristique dans l’air qui porte les sons,” said Pouillet, 
without fathoming his thought to its full depth. The deepest 
audible wave has in the mean state of the atmosphere a 
length of 10*36 metres, which is that of a column of water 
equal in weight to the whole atmosphere pressing on us. 
The highest audible sound-wave is 31 m.m., the 1 333 of 
the deepest, 1 -s- 333 being the mean centrifugality of the 
earth, and the mass of the atmosphere to that one of the 
sea, o. s. p. 
Sound is not transferable through vacuum. The waves 
2 l 2 
