are maps merely intended to give a good general idea of the appearance
of the heavens at different hours and seasons. Plate I. presents four
maps of this sort; but a more complete series of eight maps has been
published by Messrs. Walton and Maberly in an octavo work; and my own
'Constellation-Seasons' give, at the same price, twelve quarto maps (of
four of which those in Plate I. are miniatures), showing the appearance
of the sky at any hour from month to month, or on any night, at
successive intervals of two hours. But maps intermediate in character to
these and to Observatory maps are required by the amateur observer.
Such are the Society's six gnomonic maps, the set of six gnomonic maps
in Johnstone's 'Atlas of Astronomy,' and my own set of twelve gnomonic
maps. The Society's maps are a remarkably good set, containing on the
scale of a ten-inch globe all the stars in the Catalogue of the
Astronomical Society (down to the fifth magnitude). The distortion,
however, is necessarily enormous when the celestial sphere is presented
in only six gnomonic maps. In my maps all the stars of the British
Association Catalogue down to the fifth magnitude are included on the
scale of a six-inch globe. The distortion is scarcely a fourth of that
in the Society's maps. The maps are so arranged that the relative
positions of all the stars in each hemisphere can be readily gathered
from a single view; and black duplicate-maps serve to show the
appearance of the constellations.

It is often convenient to make small maps of a part of the heavens we
may wish to study closely. My 'Handbook of the Stars' has been prepared
to aid the student in the construction of such maps.

In selecting maps it is well to be able to recognise the amount of
distortion and scale-variation. This may be done by examining the spaces
included between successive parallels and meridians, near the edges and