23. Here the altitude of a star (A) is to be taken twice; firstly, when it is on the meridian in the most _southern_ point of its course, and secondly, when on the meridian in the most _northern_ point, which would be the case twelve hours later. The mean of these altitudes is the altitude of the pole, or the latitude of the place. In the example given, the star A is only 4° from the pole, which shews that it is the Pole-star, then farther from the Pole than it is now. The star F is, according to Chaucer, any convenient star having a right ascension differing from that of the Pole-star by 180°; though one having the _same_ right ascension would serve as well. If then, at the first observation, the altitude of A be 56, and at the second be 48, the altitude of the pole must be 52. See fig. 13, Plate V. 24. This comes to much the same thing. The _lowest_ or northern altitude of Dubhe ([alpha] Ursæ Majoris) may be supposed to be observed to be 25°, and his _highest_ or southern altitude to be 79°. Add these; the sum is 104; 'abate' or subtract half of that number, and the result is 52°; the latitude. 25. Here, as in § 22, Chaucer says that the latitude can be measured by the arc Z[Aries] or PN; he adds that the depression of the Antarctic pole, viz. the arc SP' (where P' is the S. pole), is another measure of the latitude. He explains that an obvious way of finding the latitude is by finding the altitude of the sun at noon at the time of an equinox. If this altitude be 38° 10', then the latitude is the complement, or 51° 50'. But this observation can only be made on two days in the year. If then this seems to be too long a tarrying, observe his midday altitude, and allow for his declination. Thus, if the sun's altitude be 58° 10' at noon when he is in the first degree of Leo, subtract his declination, viz. 20°, and the result is 38° 10', the complement of the latitude. If, however, the sun's declination be _south_, the amount of it must be added instead of subtracted. Or else we may find [Aries]A', the highest altitude of a star A' above the equinoctial, and also [Aries]A, its nether elongation extending from the same, and take the mean of the two.