The San Francisco call., 1908-11-21
HEAT OF THE EARTH

The earth is warmed chiefly from
three sources-CD the sun. supplying
heat day by day; CD coal, the sun
shine of past ages stored up; CD the
inside of the earth, which is probably
very hot
We can find the rate at which we pet
heat from the sun by measuring the
rise in temperature per second of a
body exposed to full sunshine The
heat we get out of coal can be caIc,
lated from the quantity raised-about
two million tons per day over the whole
earth The heat from the inside of the
earth may be roughly estimated by
finding how the temperature rises as
we descend deeper and deeper into the
mines.
The three sources probably give us
heat in some such proportion as 25O,OOO
to one 10 so. The sun thus gives
nearly all our warmth, and we will
consider only how this comes to us
There are three ways. says Professor
Poynting, in which heat is given by
one body to. another
Cd Conduction. when the heat trav-
els from hotter part of one body to
a cooler part, or passes from a hotter
body to a colder one in contact with it.
If we think of the temperature as slop-
ing down from a hot body to cold
one. heat is always conducted down hill.
1t never rises up the hill from O. cold to
a hot body The heat we get from the
inside Of the earth comes to US by con-
duction. The water in boiler is
warmed by conduction. The space be-
tween us and the sun is very much
colder than either the sun or the earth.
so that heat would have to come up hill
from this space to be conducted from
the sun to the earth. This does not
happen, so that no heat is conducted
to US from the sun.
CD Convection. This occurs when
hot matter is conveyed from a source
of heat to colder body to which it
gives heat by conduction when it
comes against it. This mode of heat
carriage is used in hot water and
steam pipe warming. A very small
quantity of heat may perhaps reach us
from the sun in this way, but it is at
the most exceedingly small
go Radiation. This is the way in
which heat comes from the sun. 1t is
heat in the sun. 1t is converted as it
issues from the sun into travelling
form of energy not heat. and it is con-
verted into heat again, when it falls on
a surface which absorbs it, and is
warmed by It, 1t does not appear to
need ordinary matter to carry it, 1t
passes quite freely through the almost
empty space between the sun and the
earth. 1t resembles light in travelling
in straight lines. in being reflected by
mirrors, and in being refraeted by
lenses, and it is present always with
light.
Everything we know of light shows
that light consists of waves. If we pass
a beam of white light through prism
it is split up into colors ranging from
red through yellow and green to blue
and violet-the spectrum. The red
waves are the longest about one thir-
ty-five thousandth of an inch long. and
the waves shorten as we go UP the
spectrum to the violet, which are about
one sixty thousandth of an inch long.
But these are only the waves which
affect our eyes. There are longer
waves than the red, which we can not
see, but which the thermometer feels.
and there are shorter waves than the
violet, which the Photographic plate
sees.
Very hot bodies pour out waves rang
ing from lengths much greater than
red to lengths much less than violet.
These waves. of whatever length, con-
-stitute radiation or radiant energy.
Light is only a special range of radia-
ton to which your eyes are sensitive.
The energy is heat energy before it
leaves the sun; it travels through space
in the wave form, and when it meets
an absorbing surface here it generally
turns into heat again; but it may give
the sensation of light If it falls on
your eyes. a Photographic picture If it
fall on a sensitive plate.
All bodies are always pouring out
radiation. At ordinary temperatures
perhaps the wave lengths are chiefly
about one o-thousandth of an inch
long and the shorter waves are ex-
ceedingly weak, far too weak for our
eyes to see. AS the temperature rises
the shorter waves are stronger. At
$50 degrees C. body sends out red
waves strong enough to let US see it
as red. At 800 degrees C. it sends out
all the visible rays, and see the body
white hot. The carbon of the electric
arc is about $500 degrees c.. and the
suns surface at about 5000 degrees c.