white and pied wagtails, and the dunlin. And of this remarkable change Dr. Gatke gives a minute explanation,-not, we think, to be found elsewhere:

This curious and startling phenomenon is brought about in the following manner :-Commencing below, at what afterwards marks the line of separation between the black and white markings, the colour appears at first in scarcely perceptible dots of pure black at the extreme tips of the separate barbs of each feather-the lower portion of the edge being the first to be affected, and thus acquiring a narrow border of extremely fine black specks. By degrees these edges increase in breadth until the black colour, extending towards the roots of the feathers, finally comes to be spread over their whole surface. The whole change of colouring at the particular part of the body likewise proceeds in an upward direction, so that transitional stages of the change are to be seen during the whole course of its progress.'

The remaining 420 pages of this remarkable record of birds and bird life are occupied with an elaborate description of each of the 398 visitors met with by the Doctor during his long and arduous labours through half a century. These comprise some birds rarely seen elsewhere, and a still larger number that never visit Britain. Taken in round numbers, the list of British birds, including more than 100 of rare occurrence, does not amount to more than 361, which have been thus classified :— Resident all the year

Summer visitors

Winter visitors

Capricious and rare visitors

140

63

48

110

361

Here, therefore, we have another proof, not only that Heligoland stands supreme as an observatory of bird life, but also of Dr. Gatke's unwearied labour through fifty years of patient observation. Every bird in his long catalogue is described at greater or less length with the careful and trustworthy fulness of detail that can result only from personal and original handling. The result is that his separate Account of the Birds observed in Heligoland' makes so far a complete handbook for the young student of ornithology, in a compact and convenient form. Before taking leave of Dr. Gatke, we select one final passage from the Catalogue of Birds, as an example of his general style. In describing the short-toed lark (Alauda Brachydactyla), he says:

Formerly, hardly a year passed without this pretty little lark being observed here at the end of May or June. During the time I

have been collecting it has passed through my hands about thirty times. The examples obtained in summer which undoubtedly come from Greece and Asia Minor-are always more ferruginous, especially the males, than those shot here in October and November. The predominant colour of the upper parts of the October birds is a pale dull clay-yellow, the under side being almost of a pure white, suffused on the sides of the upper breast and flanks with the colouring of the back; while, in birds coming from the south-east, the prevailing colour is of a pale ferruginous cast throughout. There is a remarkable difference in the sizes of the autumn arrivals. Some did not exceed five inches in length, while one shot in November, 1870, measured upwards of six; the wing of the first measuring 3.26 inches, and the tail 1.96 inches; that of the second being 3.78 inches, and the tail 2.56. I kept one of these little birds in a cage for more than a year; it having been stunned by a very light shot that grazed the back of its head; but it soon recovered, and became wondrously tame. In the autumn it underwent a complete moult, managed to get safely through the winter, sang heartily during the spring, but, to my regret, died at the beginning of summer. I fed it on canary seed, which, like the Lapland bunting in the next cage, it used to peel before eating; its song being much more like that of a bunting than a skylark. Heligoland is the extreme northern limit up to which it has been observed as an exceptional visitor.'

Dr. Gatke's final paragraph, in closing his labours, may well serve us in parting with a writer to whom we have owed many hours of pleasant reading.

'It is not,' he declares, without a feeling of sadness that I take leave of those dear companions of many years, whose voices, many and familiar, have come down to me like friendly greetings from the heights above during many a late hour of night spent over these pages; whilst over the skylight of the room, at once my studio and museum, their countless hosts were speeding onward to their distant homes. May these records be a welcome gift to all my fellowworkers; and with this aspiration I lay down my pen on this 19th day of May, 1890, being my seventy-seventh birthday.'

ART.

ART. IX.-1. Elementa Astronomica. Alfraganus. Amsterdam, 1669.

2. Origin and Progress of Astronomy. Narrien. London, 1833. 3. Historical Survey of the Astronomy of the Ancients. Sir G. Cornewall Lewis. London, 1862.

4. Il senso geografico-astronomico della Divina Commedia. Della Valle. Faenza, 1869.

5. Cosmografia della Divina Commedia. Vaccheri e Bertacchi. Turin, 1881.

IT

T is a matter of regret that even students of ability and culture often refuse so much as to attempt to understand Dante's astronomical references. They assume either that they are not to be understood at all, or at least not without special astronomical or mathematical training. The truth is that, as we hope to be able to show, most of them are perfectly simple and clear to anyone with a knowledge of the most rudimentary facts of astronomy, modified by the manner of their presentation on the Ptolemaic system. This, however, does not in any way increase the difficulty. Indeed it may be said to have this special advantage in the way of intelligibility and simplicity, that it treats of the motions of the heavenly bodies just as they appear to an ordinary observer, without applying any of the corrections of such prima facie appearances required by Modern Astronomy. Those who thus give up such passages in despair may well remember the warning addressed by Beatrice to Dante :'Se li tuoi diti non sono a tal nodo Sufficienti, non è maraviglia; Tanto per non tentar è fatto sodo.'

'If thy fingers are not equal to untying such a knot, it is no wonder. It has become so hard from not trying.' (Par. xxviii. 58-60.)

On the other hand many specialists-and examples of this may be found in some of the works mentioned above-have brought to bear upon these astronomical references and allusions in Dante such a wealth of technical knowledge, and have interpreted them by the aid of calculations so elaborate and minute, that they seem to forget that Dante's object was not to compose a didactic poem for teaching astronomy, but to use astronomy, as well as other branches of his very varied and extensive knowledge, in the service of poetry. He was a poet first and an astronomer afterwards.* Some of the laborious

This applies to the 'Divina Commedia,' with which most of the writers referred to are mainly, if not exclusively, concerned. The case is, of course, different with the Convito,' a designedly philosophical treatise, the astronomy of which has as yet been most inadequately discussed.

investigations

investigations just referred to would seem to suggest the reversal of that order, and to leave us with the impression that one of the most indispensable adjuncts to the study of the 'Divina Commedia' would be the Nautical Almanac.'

It is quite true that astronomy occupies an exceptionally prominent position in the great poem of Dante. It is evident from the frequency and often the elaborate character of his references to it in the poem, as well as from many lengthy discussions of astronomical phenomena in the Convito,' that he was both profoundly learned and also specially interested in this branch of science. But besides this, the subject of the 'Divina Commedia,' especially in the Paradiso,' naturally invited a large number of such references. The grades of happiness of the redeemed are associated locally with the different spheres or 'heavens' of the current astronomical science. Besides this the belief in stellar influences on human affairs and character, shared by Dante with most, if not all, thinkers of his age, imparted a special interest to the study of astronomical phenomena. Some preliminary acquaintance therefore with at least the outlines of the Ptolemaic system and phraseology is necessary for the intelligent understanding of numerous passages in Dante. That acquaintance need not, however, be extensive or minute, and in some cases, not a little knowledge,' but the advanced study of a specialist, has proved 'dangerous,' in tempting its possessor beyond the limits of the legitimate interpretation of poetical allusions.

Of the works the titles of which we have prefixed to this article, the treatise of Alfraganus is selected as a type of those (if not indeed, as is probable, the actual work itself) from which Dante's astronomical knowledge would be derived. It is an epitome of the great work of Ptolemy, by an Arabian astronomer of the ninth century. It was translated into Latin first in the twelfth century by Gerardus of Cremona (d. 1187), who was also the first translator into Latin of the Almagest itself; and again, a little later, by Johannes Hispalensis (Seville), so that it would thus have become accessible to Dante. It has been five times printed, the latest and perhaps least inaccurate edition being that which we have cited. Alfraganus is quoted by Dante in Conv. II. xiv., l. 95, as his authority for the dimensions of the planet Mercury (the passage being found in Alfr. c. xxii.). Again he quotes his work in Conv. II. vi., 1. 134, under the title by which it is sometimes known, as 'Libro dell' aggregazione delle stelle.' Besides this, most of the astronomical data, and even sometimes the comparisons and illustrations, given by Dante are found totidem verbis in Alfraganus. This, therefore, Vol. 187.-No. 374.

2 L

or

or some similar epitome of the Almagest, he evidently used freely, and that it was in fact this particular work is made most probable by these definite citations of it.

To the elaborate and learned works of Professor Narrien and Sir G. C. Lewis, which follow next on our list, we wish to acknowledge our very large obligations. We have relied upon them for most of the facts relating to the early history of astronomy adduced in the following pages. After this we shall not consider it necessary to repeat this acknowledgment in respect of details from time to time. The former of these works deals mainly with the historical aspect of the subject. The latter in addition developes the mathematical problems involved in the theories of the early astronomers. Both writers, it will be understood, treat of the subject in its general aspect and without any reference to the works of Dante, who is never once mentioned by them in this connexion. The remaining works are taken as a type of the specialist learning that has been brought to bear upon this aspect of Dante's poem, and the list is one which might be considerably added to.

As our object is to help readers of Dante, whom Plato would describe as 'ayewμéтρηтоi,' to understand his astronomical references and allusions, we may be pardoned if we sometimes seem to err on the side of over-explanation of points which appear simple enough to those who are even moderately versed in the rudiments of astronomy.

The correctness of many of the results obtained under the Ptolemaic system is quite astonishing, when we consider the imperfection of their instruments of observation and of their timekeepers, the paucity of recorded observations for comparison, and the falsity of the fundamental assumptions of the system. The last-mentioned defect, however, is not so serious a drawback as it might at first sight appear. The apparent results would not be affected by the falsity of the hypothesis by which they were explained. A familiar illustration of this may be given from common experience. When one of two trains is stationary and the other moving slowly and smoothly it is often impossible to detect by the sight whether we are ourselves in the moving or the stationary train. So, whether the sun revolves about the earth or the earth about the sun, there is absolutely no difference in the resulting visible phenomena, which are the subject of astronomical research and record. This is noticed by Cicero in respect of the revolution of the earth about its axis, and that of the heaven about the stationary earth.

We propose to use the term 'Ptolemaic' as a convenient designation of the geocentric or pre-Copernican system of astronomy

generally.