The most important part of the 'astrolabe planisphere' consisted of a somewhat heavy circular plate of metal from four to seven inches in diameter, which could be suspended from the thumb by a ring (i. 1), working with such freedom as would allow the instrument to assume a perfectly perpendicular position (i. 2). One side of the plate was perfectly flat, and was called the _back_. This is represented in Fig. 1. On it was described a number of concentric rings, marked with various divisions, which may be readily understood from the figure. Beginning at the outermost ring, the first two represent the ninety degrees into which each quadrant of a circle can be divided (i. 7). The next two represent the signs of the zodiac, each subdivided into thirty degrees (i. 8). The next two represent the days of the year, and are rather difficult to mark, as the circle has, for this purpose, to be divided into 365¼ equal parts (i. 9). The next three circles shew the names of the months, the number of days in each, and the small divisions which represent each day, which coincide exactly with those representing the days of the year (i. 10). The two innermost rings shew the saints' days, with their Sunday-letters. Thus, above the 21st of December is written 'Thome,' i.e. St. Thomas's day, its Sunday-letter being E; the rest can easily be traced by the tables in a Prayer-book (i. 11). These may be thus briefly recapitulated:-- 1 and 2. Circles of degrees of the quadrant and circle. 3 and 4. Circles of the zodiacal signs, with their degrees. 5 and 6. Circles of the days of the year, with their numbers. 7, 8 and 9. Circles of the months, with their days and numbers of the days. 10 and 11. Circles of saints' days, with their Sunday-letters. Within all these, are the Scales of Umbra Recta and Umbra Versa, in each of which the scale is divided into twelve equal parts, for the convenience of taking and computing altitudes (i. 12). This primitive and loose method of computation has long been superseded by the methods of trigonometry. Besides these circles, there is a perpendicular line, marking the South and North points, and a horizontal line from East to West.