328 
POPULAR SCIENCE REVIEW. 
sun originally communicated its force to tlie plant ; tlie 
plant took up the force, condensed it, and held it. The 
animal receiving the force from the vegetable, disengages it, 
and animal motion is the result. This force, however, 
when brought into play, were spent without effect, unless it 
were counterbalanced by the force exerted upon it by the 
earth. By this latter agency man is enchained to" the planet, 
and he feels the influence every day he lives. When a sick 
man says, “ I am weak, and must lie down,” he announces in 
direct terms the action of the earth upon his organism. In 
scientific phraseology he is expressing that he is being 
subjected to the influence of gravitation; in other words, that 
is to say, he is not charged with so much sun force that he 
can resist the persistent and powerful earth force. 
It is the same, physically, with a steam-engine. In the 
motion of the engine we observe force liberated from coal ; 
radiating force, once in the sun, anon laid up in the plant, 
now set free, and producing through matter movement, or 
motion made indirectly visible. When the engine stops, we 
see again the influence of earth force ; we see the earth that 
for a time has been conquered, reasserting its power. 
It has been customary to look upon these two forces as 
standing to each other in the relation of activity and nega- 
tivity, and the words (i action ” and “ inertia ” have been 
employed to distinguish between them. This hypothesis is 
natural, but it is not strictly correct, for both forces are, in 
truth, active, and both admit of being applied to make motion 
visible. Thus, when I wind up a clock- weight, I have truly 
expended so much sun force in the effort; but I leave tho 
instrument, the clock, and straightway the weighted body I 
have raised, coming under the influence of the planet, descends 
and brings into motion the machinery by which the hands of 
the clock are made to move. In like manner, when I place a 
wheel in a descending current of water, as in the water-mill, 
I use actually the force of the earth, the weight of the water, 
to make the wheel revolve. The water which previously had 
been raised by sun force, descends by earth force, and gives 
the motion to the machine. It was the recognition, doubtless, 
of these facts that led the great Buffon to lay down the 
physical law or axiom, that all the powers of nature with 
which we are conversant, are reducible to two : that which 
causes heat, and that) which causes weight. Thus both forces 
may be active, but there is this always to be remembered, 
that in every case the sun force is primarily called into 
play. The earth can do nothing except hold things to itself 
until it is resisted by the force of the sun. 
Ho subject has been more keenly debated amongst philoso- 
