ing the water, because if the cylinder is
surrounded by blue glass which absorbs the red rays and prevents their
passage into the water, the temperature of the water begins to fall.
That the other light rays have a small share would have been clear
from the preceding Section.

All the energy of the sunshine which falls upon the cylinder, both as
heat and as light, is absorbed in the form of heat, and the total
amount of this energy can be calculated from the increase in the
temperature of the water. The energy which heated the water would have
passed onward to the surface of the earth if its path had not been
obstructed by the cylinder of water; and we can be sure that the
energy which entered the water and changed its temperature would
ordinarily have heated an equal area of the earth's surface; and from
this, we can calculate the energy falling upon the entire surface of
the earth during any one day.

Computations based upon this experiment show that the earth receives
daily from the sun the equivalent of 341,000,000,000 horse power--an
amount inconceivable to the human mind.

Professor Young gives a striking picture of what this energy of the
sun could do. A solid column of ice 93,000,000 miles long and 2-1/4
miles in diameter could be melted in a single second if the sun could
concentrate its entire power on the ice.

While the amount of energy received daily from the sun by the earth is
actually enormous, it is small in comparison with the whole amount
given out by the sun to the numerous heavenly bodies which make up the
universe. In fact, of the entire outflow of heat and light, the earth
receives only one part in two thousand million, and this is a very
small portion indeed.

139. How Light and Heat Travel from the Sun to Us. Astronomers tell
us that the sun--the chief source of heat and light--is 93,000,000
miles away from us; that is, so far distant that the fastest express
train would require about 176 years to reach the sun. How do heat and
light travel through this vast abyss of space?

[Illustration: FIG. 90.--Waves formed by a pebble.]

A quiet pool and a pebble will help to make it clear to us. If we
throw a pebble into a quiet pool (Fig. 90), waves or ripples form and
spread out in all directions, gradually dying out as they become more
and more distant from the pebble. It is a strange fact that while we
see the ripple moving farther and farther away, the particles of water
are themselves not m