expended his labour upon very inferior materials, and has been sometimes misled by the badness of those MSS. to which alone he had access1.

Besides his print of Chaucer's Astrolabe, Mr. Brae has reprinted some curious and interesting critical notes of his own, and has added some essays on Chaucer's 'prime,' on 'the Carrenare,' and 'shippes opposteres.' To all that he has done I am much indebted.

works upon, and

I have had neither

$27. WORKS ON THE SUBJECT. The descriptions of, the astrolabe, are numerous. time nor inclination to make researches into the subject; for which reason I here note the names of a few books which may be examined by the curious reader.

In his Universal Lexicon, Zedler explains that astrolabes are of two kinds, 'universal' and 'particular.' He speaks of the astrolabes (1) of Gemma Frisius; see Petri Apiani Cosmographia, per Gemmam Phrysium restituta; (2) of Johan de Rojas, a Spaniard, A. D. 1550; (3) of De la Hire the elder, professor of mathematics at Paris, A.D. 1702; (4) of Johannes Stoflerinus (or Stöffler), A.D. 1510. The last of these varied from the others in adopting a different and more convenient system of projection, viz. that upon the plane of the equator, or one parallel to it, the eye being in the antarctic pole, and the arctic pole being made the centre of the instrument. This projection is the same as that which was used by Ptolemy, and it is adopted in the diagrams which accompany Chaucer's treatise in some of the MSS. It should be observed here that the term 'astrolabe' alone is vague; it was originally a general name for any circular instrument used for observation of the stars; but in the sixteenth and seventeenth centuries it was restricted to the particular kind called the 'Astrolabe Planisphere,' or astrolabe on a flat surface, in which sense alone the word is used throughout this volume. See the English Cyclopaedia, Arts and Sciences, s. v. Astrolabe.

The simplest work is that by Stöffler or Stoflerinus, as he calls himself; see also Gemma Frisius, Metius, Clavius Bambergensis, the Cursus Mathematicus of Dechales, vol. iv. p. 161, Delambre's History of Astronomy, and other works. The plates in Metius

1 In my edition of the 'Astrolabe' for the Early Eng. Text Society (1872), I have inserted a large number of examples of strange blunders in the printed editions.

are most exquisitely engraved, and on a large scale, and give a better representation of the instrument than any others that I have seen.

One of the MSS., viz. MS. E., refers to an astrolabe belonging to Merton College, Oxford'. There is a very nice one, made of brass, and by a Dutch engraver, in the library of King's College, Cambridge. It has several discs or plates, or, as Chaucer calls them, 'tables". Of this instrument the same library contains a written description, with some account of the problems it will solve, and an investigation of its probable date, by H. Godfray, Esq., of St. John's College.

There is a book entitled 'A verie briefe and most plaine description of Mr. Blagrave his Astrolabe,' &c., by Mr. Blundevill; London, printed by William Stansby. But it turns out to be of little practical assistance, because Blagrave's astrolabe was on a different principle.

§ 28. DESCRIPTION OF THE Astrolabe PlanISPHERE. There is not, however, much need of reference to books to understand what the astrolabe used by Chaucer was like. The instrument may be readily understood from a brief description, and from the Plates in this volume.

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

1 There are two astrolabes in Merton College, besides a plate exhibiting astronomical tables. These are all described in a paper entitled 'Remarks on an Astrolabe belonging to F. A. Hyett, Esq.,' written by my friend Robert Taylor, M.A., and printed in the Transactions of the Bristol and Gloucestershire Archæological Society, vol. xii. Mr. Taylor further describes two Astrolabes in the British Museum.

"This word has several senses in Chaucer. It means (1) the discs of an astrolabe; (2) a set of tablets; (3) astronomical tables; and (4) the game of 'tables.'

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 365 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.

The other side of the plate, called the front, and shewn in Fig. 2, had a thick rim with a wide depression in the middle (i. 3). The rim was marked with three rings or circles, of which the outermost was the Circle of Letters (A to Z) representing the twenty-four hours of the day, and the two innermost the degrees of the quadrants (i. 16). The depressed central portion of the plate was marked only with three circles, the 'Tropicus Cancri,' the Equinoctialis,' and the 'Tropicus Capricorni' (i. 17); and with the cross-lines from North to South, and from East to West (i. 15). But several thin plates or discs of metal were provided, which were of such a size as exactly to drop into the depression spoken of. The principal one of these, called the 'Rete,' is shewn in Fig. 2. It consisted of a circular ring marked with the zodiacal signs, subdivided into degrees, with narrow branching limbs both within and without this ring, having smaller

branches or tongues terminating in points, each of which denoted the exact position of some well-known star. The names of these stars, as Alhabor,' 'Rigel,' &c., are (some of them) written on the branches (i. 21). The 'Rete' being thus, as it were, a skeleton plate, allows the Tropicus Cancri,' &c., marked upon the body of the instrument, to be partially seen below it. Another form of the 'Rete' is shewn in Fig. 9, and other positions of the Rete in Fig. 11 and Fig. 12. But it was more usual to interpose between the 'Rete' and the body of the instrument (called the 'Mother') another thin plate or disc, such as that in Fig. 5, so that portions of this latter plate could be seen beneath the skeleton-form of the 'Rete' (i. 17). These plates are called by Chaucer 'tables,' and sometimes an instrument was provided with several of them, differently marked, for use in places having different latitudes. The one in Fig. 5 is suitable for the latitude of Oxford (nearly). The upper part, above the Horizon Obliquus, is marked with circles of altitude (i. 18), crossed by incomplete arcs of azimuth tending to a common centre, the zenith (i. 19). The lower part of the same plate is marked with arcs denoting the twelve planetary hours (i. 20).

At the back of the astrolabe revolved the 'rule,' made of metal, and fitted with sights, represented in Fig. 3 (i. 13). At the front of it revolved the 'label,' represented in Fig. 6 (i. 22).

All the parts were held together by the central pin (Fig. 4) which passed through the holes in the 'moder,' plates, 'Rete,' rule, and label', and was secured by a little wedge (i. 14), which was sometimes fancifully carved to resemble a horse (Fig. 7).

Another 'table' or disc is shewn in Fig. 14, and was used for ascertaining the twelve astrological houses.

§ 29. USES OF THE ASTROLABE PLANISPHERE. I here briefly enumerate such principal uses of the instrument as are mentioned by Chaucer.

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

'Pertuis: m. A hole. Pertuis de l'Araigne, the centre of an Astrolabe; the hole wherein all the tables thereof are, by a pin or naile, joined together.'Cotgrave's French Dictionary.

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).

By the use of the Umbra Recta on the back of the instrument, we can take the altitude of an accessible object by a single