in the nebula.  The history of this star is a remarkable one.  It suddenly kindled from invisibility into brilliancy.  How is a change so rapid in the lustre of a star to be accounted for?  In a few days its brightness had undergone an extraordinary increase.  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 expands, and when it expands it becomes lighter.  This lighter air rises, like wood plunged into water, through the heavier air overhead.