ease. Of course,
this does not tell us for certain that the star lay in the glowing
gas; but the most rational explanation that I have heard offered of
this occurrence is that due, I believe, to my friend Mr. Monck. He has
suggested that the sudden outbreak in brilliancy might be accounted
for on the same principles as those by which we explain the ignition
of meteors in our atmosphere. If a dark star, moving along with
terrific speed through space, were suddenly to plunge into a dense
region of the nebula, heat and light must be evolved in sufficient
abundance to transform the star into a brilliant object. If,
therefore, we knew the distance of this star at the time it was in
Andromeda, we should, of course, learn the distance of that
interesting object. This has been attempted, and it has thus been
proved that the Great Nebula must be very much further from us than is
that star of whose distance I attempted some time ago to give you a
notion.

We thus realize the enormous size of the Great Nebula. It appears that
if, on a map of this object, we were to lay down, accurately to scale,
a map of the solar system, putting the sun in the centre and all the
planets around their true proportions out to the boundary traced by
Neptune, this area, vast though it is, would be a mere speck on the
drawing of the object. Our system would have to be enormously bigger
before it sufficed to cover anything like the area of the sky included
in one of these great objects. Here is a sketch of a nebula, Fig. 10,
and near I have marked a dot, which is to indicate our solar system.
We may feel confident that the Great Nebula is at the very least as
mighty as this proportion would indicate.

[Illustration: FIG. 10. THE SOLAR SYSTEM AS COMPARED WITH A GREAT
NEBULA.]

RAIN AND SNOW

(FROM THE FORMS OF WATER.)

BY JOHN TYNDALL.

Oceanic Distillation.

[Illustration: SNOW CRYSTALS.]

At the equator, and within certain limits north and south of it, the
sun at certain periods of the year is directly overhead at noon. These
limits are called the Tropics of Cancer and of Capricorn. Upon the
belt comprised between these two circles the sun's rays fall with
their mightiest power; for here they shoot directly downwards, and
heat both earth and sea more than when they strike slantingly.

When the vertical sunbeams strike the land they heat it, and the air
in contact with the hot soil becomes heated in turn. But when heated
the air ex