Evening star. [volume] (Washington, D.C.) 1854-1972, 1906-04-29

moving Photographic plate. and the pho
tograph upon the plate is a zigzag line,
which gees to and fro as the bolometer is
warmed or cooled by changing sun rays.
k
In the other building Is installed another
delicate instrument-the ''pyrheiometer
for measuring the rate at which the suns
heat is received at the earths surface. A
bucket of water is warmed by the sun
much faster on Il mountain top than at the
brink of the sea, because upon the noun
tain there is less air between the bucket
and the to cut off the rays by absorb
! ton. Since all of the suns rays, whether
absorbed at the earth's surface or in the
air above it. are vital to climate and vege-
tation it IS important to determine how
much of these rays are thus lost in the air.
This amount is found to vary greatly at diff
fefent times. not only because of clouds or
invisible moisture in the air, but because of
dust and smoke therein. If one pyrhelio
meter were carried up to the outer limit OF
our atmosphere while another were sta
toned at sea level comparison of their
records would show precisely how much OF
the suns heat is thus absorbed by the
earth's envelop of air. This being impossible
the measurements must be made the
rays of the sun at different hours of the
day, for as Ii approaches the horizon the
sun shines through thicker and thicker lay
ers of air. The pyrheliometer can give such
measurements only for the rays of light In
all their completeness, but a sunbeam is
composed of the five colors seen in the rain
bow. and these may each be subdvided into
an infinite number of shades grading one
into another. Now the air scatters and alb
serbs all these colors differently, and to
make the measurements complete the sun
beams must be passed through prism and
thus split into these elementary colors ar
ranging themselves in that continuous band
which scientists call the spectrum. Tiese
rays of the different colors are run slngly
through the ''bolometer first instrument
mentioned-which measures their energies
separately and shows their respective losses
during the passage through the air. In
autumn the air Is. as we all know, generally
more transparent than In spring and sum-
mer, but it sometimes happens that whole
years are found by these delicate measure
ments to differ from others in the trans
parency of the air. 1t is thus discovered
that for two years during 1901 ana 1917, the
air allowed much more heat TO pass through
it than during the first eight months OF
1893 after which the transparency In
creased to about it's former state. 1t Is
thought possible that this change was
caused by the great volcanic eruption or
Mount Pelee, which sent thousands of tons
of dust into the air
speaking generally, 1t Is found that the
suns BEAM in passing entirely through
our atmosphere, to sea level loses about
falt of its heat in air, but most OF this less
is made up to US on clear day by the
light coming from all parts of the sky.
But clouds, on the other hand, reflect away
about three-fourths of the sunlight that
reaches them; and as the earth Is 60 per
cent covered with clouds, 1t follows that
II the earth did not have this pro-
tecting cloudy envelope 1t would get nearly
twice as much heat from the and the
temperature upon its surface would be fully
so degrees warmer than 1t is a. present.
*.
In addition to these changes in our at
mospheric transmission of the sunlight TO
ha eli ri In the
suns actual radiation of heat. By meas
uring the total amount Of sunlight reaching
the surface of the earth, and adding to 1t
the amount estimated as lost in coming