288 FRAGMENTS OF SCIENCE. 
ish the velocity of the earth's rotation.* Supposing then 
that we turn a mill by the action of the tide, and produce 
heat by the friction of the millstones; that heat has an 
origin totally different from the heat produced by another 
mill which is turned by a mountain stream. The former- 
is produced at the expense of the earth's rotation, the 
latter at the expense of the sun's radiation. 
The sun, by. the act of vaporization, lifts mechanically 
all the moisture of our air, which when it condenses falls in 
the form of rain, and when it freezes falls as snow. In 
this solid form it is piled upon the Alpine heights, and fur- 
nishes materials for glaciers. But the sun again inter- 
poses, liberates the solidified liquid, and permits it to roll 
by gravity to the sea. The mechanical force of every 
river in the world as it rolls toward the ocean, is drawn 
from the heat of the sun. No streamlet glides to a lower 
level without having been first lifted to the elevation from 
which it springs by the power of the sun. The energy of 
winds is also due entirely to the same power. 
But there is still another work which the sun performs, 
and its connection with which is not so obvious. Trees 
and vegetables grow upon the earth, and when burned 
they give rise to heat, and hence to mechanical energy. 
Whence is this power derived? You see this oxide of iron, 
produced by the falling together of the atoms of iron and 
oxygen; you cannot see this transparent carbonic acid gas, 
formed by the falling together of carbon and oxygen. 
The atoms thus in close union resemble our lead weight 
while resting on the earth; but we can wind up the weight 
and prepare it for another fall, and so these atoms can be 
wound up and thus enabled to repeat the process of com- 
bination. In the building of plants carbonic acid is the 
material from which the carbon of the plant is derived; 
and the solar beam is the agent which tears the atoms 
asunder, setting the oxygen free, and allowing the carbon 
to aggregate in woody fiber. Let the solar rays fall upon a 
surface of sand; the sand is heated, and finally radiates 
away as much heat as it receives; let the same beams fall 
upon a forest, the quantity of heat given back is less than 
the forest receives; for the energy of a portion of the sun- 
beams is invested in building the trees. Without the sun 
* Kant surmised an action of this kind. 
