The _back_ (Fig. 1) shews at once the degree of the zodiac answering to every day in the year (ii. 1). The altitude of the sun can be taken by the 'Rule,' elevated at the proper angle (ii. 2). If the Rete be properly adjusted to this altitude, we can thus tell the hour of the day (ii. 3). The duration of twilight can be calculated by observing when the sun is 18° below the horizon (ii. 6). Observe the times of sunrise and sundown, and the interval is the 'artificial day' (ii. 7). This day, with the duration of morning and evening twilights added to it, is called the 'vulgar day' (ii. 9). The plate in Fig. 5 shews the planetary hours (ii. 12). The placing of the sun's degree on the South-line gives the sun's meridian altitude (ii. 13), and conversely (ii. 14). The back of the instrument can shew what days in the year are of equal length (ii. 15). The degree of the zodiac which souths with _any_ star can be ascertained by observing two altitudes of the star; but the observations must be made when the star is _very near_ the meridian (ii. 17). If the star be marked on the Rete, the said degree is easily found by use of the Rete (ii. 18). We can also find with what degree of the zodiac the same star rises (ii. 19). The use of the Rete also shews the declination of every degree in the zodiac (ii. 20). We can always tell for what latitude a disc such as that in Fig. 5 is constructed, by properly examining it (ii. 21). The latitude of any place can be found by two observations of the altitude of the Pole-star (ii. 23); or of any circumpolar star (ii. 24); or by observing the sun's meridional altitude (ii. 25). The Rete also tells us the 'ascensions of signs,' or how many degrees of the equinoctial circle pass the meridian with a given sign (ii. 27); as also the 'oblique ascensions' of the same (ii. 28). The astrolabe can also be used to discover (but only in an imperfect and approximate manner) the four cardinal points of the compass (ii. 29). We can also compare the altitude of a planet with that of the sun (ii. 30). We can find in what part of the horizon the sun rises (ii. 31); and in what direction to look for a conjunction of the sun and moon (ii. 32); also near what point of the compass the sun is at any given hour (ii. 33). The moon's observed altitude will shew her longitude (ii. 34). We can tell, from two observations of a planet properly made, whether the planet's movement is direct or retrograde (ii. 35). The disc shewn in Fig. 14 helps to shew the 'equations of houses' (ii. 36). The four cardinal points can be found _without_ an astrolabe, by an experiment properly conducted (ii. 38). The astrolabe can be used to find the degree of the zodiac with which any planet ascends, even when the planet is not situated in the ecliptic (ii. 40).