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. Again, it must be noted that, although the maps are necessarily arranged in a certain order, there is in reality no first or last in the series. The map numbered I. follows the map numbered XII. in exactly the same manner that the latter follows the map numbered XI. The maps form a circular series, in fact. The only reason for numbering the maps as at present, is that the map numbered I. Happens to exhibit the aspect of the sky at a convenient hour on the night of January 1st. It will be found that the dates follow on with intervals of seven or eight days right round the year, the end of the year falling in the left-hand column of the table under Map I., while the beginning of the year is in the right-hand column of the same map.[*] [Footnote *: It may be mentioned in passing, that the dates have not been thrown in so as to fall regularly round the year, but correspond with the variations due to the earth's variable motion round the sun.]