, 1912-03-30

that certainly brings Us a step forward in
cur study bat before taking The nawt -
let us ask how is the fact OF the dAcrosaDf
temperature as we ascend to be- a.c1nmt
for? We find a good answer br ProfI"|
Looms, Ill his "Treatise on Metoorolonvn
but as 1t IS gwen Ill technical lau0uaoe
VIII, Tor the sake of our young readers trv
to put 1t into a familiar form. In substanoo
the explanation is this: The sun. heats tho
earth's surface and this in turn warns tL1.
part OF the air resting upon it, partly bv what
1t throws off or radiates into the mir L,,t
mainly Ill this way, that the hot mir r;.os
ana 50 gives place to cooler air. which in
turn becomes warm aud ascends. The heat
tAus causes the ar near the surface tn ris4
ana as 1t rises 1t expands, because less air
presses on 1T from above. The expansion.
causes the cold "Ihls principle" says Pho
feSsof Looms "inay be proved experiment
tally by placing u thermometer under the.
receiver Of an air-pulnp and rapidly exhaust
Ing the air,. when the thermometer iudicntes
IL clLn!nuClou Of sensible temperatures. UDon
~readmlttlnS the air the thermometer rises To,
it's former height"
So far, then, we have learned that the sim,
sends his rays from u vast distance on to-
the sheet OF lce-crystalS and on this sheet
we see the haios.
What connection is there 6eteCee2a the ire.
crystals ae@ The halos Tuis question brings
Us to more difficult considerations than anv
OF the foregoing. We might get rid OF it by
saying that the light of the sun is refracted
through the ice-crystals and 50 produces
both the forms and the colours of the halos;,
it would be true enough, but of what use,
would such a statement be unless we have
some simple but clear notions of refruction
and how it acts? So suppose that we break
up our question aud ask first
What is refrUction2 Literally it means.
6~seakUtg-b0cE and the name is applied to the
bend which u ray OF light suffers as it passes.
from one transparent substance to another
of different density. If this statement isJ
quite new to us we may not see ut first
glance what it means, but with u little effort
we shall see as we go along. Aud the
effort is we'll worth the WHILE for we
owe many of our greatest gifts. including
sight itself. t. the refraction of light.
Ii we think OF air and water we are think
ing OF two substances of different denslties.
Tn this case both happen to be- full, but
there can be- no doubt that water is denser
than air. Suppose we try Il little expert
ment with these, substances: we take nA
empty wasll-bnsin and put u coin in the
bottom, then, seating ourselves in such -
ROSENON OANN oAD ON0o ~ Wv CT1AA IbOO AlAO6'
the coin, we get someone to pour water into
the basin so gently that tue coin is NOT
moved. We then see that the bottom OF
the basin seems to rise and the coin comes
into view. What has happened is this
the light from the coin, which enables us to-
see it, has been beut downwArls on passing
from the water into the air hence we are.
able to see what was previously hidden.
9lass has greater density than air. When
light falls pe1pendculnIIy on to a sheet
of glass it goes through it without suffering
refraction, but when the direction of the
light is siopng, it is, on entering the glass,
bent towards the perpendicular, and on
leaving the sheet on the other side it is
again refractet and continues onward
in the same direction as before. This is
shown in the diogram, where c. is the
glass sheet, 'P' perpendicular ray and s u
slanting ray, leFracted at 't' and r..