It is clear, then, that what the learner wants is a set of maps
specially constructed to show him in what part of the sky the
constellations are to be looked for. He ought on any night of the
year to be able to turn at once to the proper map, and in that
map he ought to see at once what to look for, toward what point
of the compass each visible constellation lies, and how high it
is above the horizon. And, if possible (as the present work shows
is the case), _one_ map ought to suffice to exhibit the aspect of
the whole heavens, in order that the beginner may not be confused
by turning from map to map, and trying to find out how each fits
in with the others.

It is to fulfil these requirements that the present maps have been
constructed. Each exhibits the aspect of the whole sky at a given
day and hour. The circumference of the map represents the natural
horizon, the middle of the map representing the part of the sky
which lies immediately overhead. If the learner hold one of these
maps over his head, so as to look vertically upwards at it, the
different parts of the horizon marked in round the circumference
being turned towards the proper compass points, he will see the
same view of the heavens as he would if he were to lie on his back
and look upwards at the sky, only that the map is a planisphere
and the sky a hemisphere.

But although this illustration serves to indicate the nature of
the maps, the actual mode of using them is more convenient.

Let it first be noted that properly speaking the maps have neither
top, bottom, nor sides. Each map may be held with any part of the
circumference downward: then the centre of the map is to be looked
upon as the top for that part of the circumference. The portion
of the map lying beneath the centre represents the portion of the
sky lying between the point overhead and a certain part of the
horizon--the part in fact corresponding to the particular part
of the circumference which is turned downwards. Thus if on any
night we wish to learn what are the stars towards the north, we
look for the map corresponding to that night. At the hour named
the stars toward the north will be those shown between the centre
of the map and the top; and, of course, we hold the map upside
down so as to bring the centre above the northern part of the
circumference.

But this matter will be more clearly understood by comparing the
account of any of the accompanying maps with the map itself.

A