a vast hollow sphere, and this sphere rotated about an axis.  This axis, in our latitude, is inclined about 51-1/2 deg. to the horizon.  Of course only one end of this imaginary axis can be above our horizon.  This end lies very near a star which it will be well for us to become acquainted with at the beginning of our operations.  It lies almost exactly towards the north, and is raised from 50 deg. to 53 deg. (according to the season and hour) above the horizon.  There is an easy method of finding it.

We must first find the Greater Bear.  It will be seen from Plate 1, that on a spring evening the seven conspicuous stars of this constellation are to be looked for towards the north-east, about half way between the horizon and the point overhead (or zenith), the length of the set of stars being vertical.  On a summer’s evening the Great Bear is nearly overhead.  On an autumn evening he is towards the north-west, the length of the set of seven being somewhat inclined to the horizon.  Finally, on a winter’s evening, he is low down towards the north, the length of the set of seven stars being nearly in a horizontal direction.

Having found the seven stars, we make use of the pointers [alpha] and [beta] (shown in Plate 1) to indicate the place of the Pole-star, whose distance from the pointer [alpha] is rather more than three times the distance of [alpha] from [beta].

Now stand facing the Pole-star.  Then all the stars are travelling round that star in a direction contrary to that in which the hands of a watch move.  Thus the stars below the pole are moving towards the right, those above the pole towards the left, those to the right of the pole upwards, those to the left of the pole downwards.

Next face the south.  Then all the stars on our left, that is, towards the east, are rising slantingly towards the south; those due south are moving horizontally to the right, that is, towards the west; and those on our right are passing slantingly downwards towards the west.

It is important to familiarise ourselves with these motions, because it is through them that objects pass out of the field of view of the telescope, and by moving the tube in a proper direction we can easily pick up an object that has thus passed away, whereas if we are not familiar with the varying motions in different parts of the celestial sphere, we may fail in the attempt to immediately recover an object, and waste time in the search for it.

The consideration of the celestial motions shows how advantageous it is, when using an alt-azimuth, to observe objects as nearly as possible due south.  Of course in many cases this is impracticable, because a phenomenon we wish to watch may occur when an object is not situated near the meridian.  But in examining double stars there is in general no reason for selecting objects inconveniently situated.  We can wait till they come round to the meridian, and then observe them more comfortably.  Besides, most objects are higher, and therefore better seen, when due south.