THE CONSTITUTION F NA TUBE. 3 
But suppose the water withdrawn; the action at a dis- 
tance would then cease, and, as far as the sense of touch is 
concerned, the wader would be first rendered conscious of 
the motion of the wheel by the blow of the paddles. The 
transference of motion from the paddles to the water is 
mechanically similar to the transference of molecular 
motion from the heated body to the ether; and the prop- 
agation of waves through the liquid is mechanically 
similar to the propagation of light and radiant heat. 
As far as our knowledge of space extends, we are to con- 
ceive it as the holder of the luminiferous ether, through 
which are interspersed, at enormous distances apart, the 
ponderous nuclei of the stars. Associated with the star 
that most concerns us we have a group of dark planetary 
masses revolving at various distances round it, each again 
rotating on its own axis; and, finally, associated with 
some of these planets we have dark bodies of minor note 
the moons. Whether the other fixed stars have similar 
planetary companions or not is to us a matter of pure con- 
jecture, which may or may not enter into our conception 
of the universe. But probably every thoughtful person be- 
lieves, with regard to those distant suns, that there is, 
in space, something besides our system on which they 
shine. 
From this general view of the present condition of space, 
and of the bodies contained in it, we pass to the inquiry 
whether things were so created at the beginning. Was 
space furnished at once, by the fiat of Omnipotence, with 
these burning orbs? In presence of the revelations of 
science this view is fading more and more. Behind the 
orbs we now discern the nebulee from which they have 
been condensed. And without going so far back as the 
nebulae, the man of science can prove that out of common 
non-luminous matter this whole pomp of stars might have 
been evolved. 
The law of gravitation enunciated by Newton is, that 
every particle of matter in the universe attracts every 
other particle with a force which diminishes as the square 
of the distance increases. Thus the sun and the earth 
mutually pull each other; thus the earth and the moon 
are kept in company; the force which holds every respec- 
tive pair of masses together being the integrated force of 
their component parts, Under the operation of this force 
