Art. 8.-SIR DAVID GILL AND RECENT ASTRONOMY.

A History and Description of the Royal Observatory, Cape of Good Hope. By Sir David Gill, K.C.B. London: H.M. Stationery Office, 1913.

IN the autumn of 1913, shortly before the death of Sir David Gill, his book, called 'A History and Description of the Royal Observatory, Cape of Good Hope,' was published by order of the Lords Commissioners of the Admiralty in obedience to His Majesty's command. It is a large quarto, of which 136 pp. give, with many excellent plates, a description of the splendid instruments designed for and erected at the Cape under the supervision of Sir David Gill, who, from 1879 to 1907, held the post of H.M. Astronomer there. The introductory history, extending to 190 pages, records the work done at the Observatory, both before and after his appointment.

The value of this book to astronomers, professional or amateur, is incalculable. The death of the author on January 24, 1914, so soon after he had thus summarised a life's work in those pages, ensures the interest of a large outside public in the extraordinary developments. made in the science of astronomy during his lifetime. It also justifies the reviewer in glancing a little beyond the written pages to speak of the beautiful mind and the inspiring ideals of this great man, who will live in the hearts of all who knew him, not only as among the greatest of astronomers, but also as one of the noblest and most lovable of men.

During the two centuries that followed the publication of Sir Isaac Newton's 'Principia,' the greatest delight of astronomers was to watch the members of the solar system, to measure their positions with the highest precision, to learn to foretell their movements, and to test the universality of the law of gravitation in every case of apparent deviation from that law. The discovery of Neptune in 1846, before it had ever been seen, by its disturbing action upon the planet Uranus, was the crowning triumph of that phase of astronomical research. After this, attention came to be directed more and more to the so-called fixed stars. The revolutions, in many cases, of a pair of stars round a common centre of

gravity, exactly as in the solar system, seemed to extend the law of gravitation to stellar systems. Even the variability in the brightness of some stars could be attributed to the mutual eclipses of two stars, visible only as one, while revolving round each other; and the spectroscope added new evidence of the existence of such stellar systems subject to the force of gravitation. Then the direction and amount of the minute proper motions of some stars were measured, and, after their distances had been, with great difficulty, found in some cases, the actual velocity indicated by the proper motion could be expressed in miles per second. Gradually data accumulated for building up a stellar universe in perspective, and detecting the motion of our sun among the stars, and discovering the existence of great swarms of stars, the members of each swarm all moving in the same direction.

In later years the invention of the spectroscope has helped to divert attention from the solar to the stellar systems. This instrument enables the constitution of the stars to be discovered by the colours of the light emitted. The appearance of the spectrum as a ribbon of colours from red to violet along its length is too well known to need description; also the absence of certain colours, as indicated by black lines crossing the ribbon. The spectrum of the star Arcturus has hundreds of these black lines corresponding in position exactly with the lines shown in the spectroscope by glowing iron vapour, because iron is present in the star Arcturus. So with other chemical elements and with other stars. This study has originated the new astronomy dealing with the physical constitution of all the heavenly bodies.

But the spectroscope has another wonderful application. Sometimes the black lines representing iron in the spectrum of a star like Arcturus are shifted slightly to one side or other of the corresponding lines in the spectrum of glowing iron vapour; and this, when interpreted by a knowledge of the theory of light, enables astronomers to say whether the distance from us to any star is increasing or diminishing, and to measure the speed in miles per second. This marvellous faculty has given a new impetus to the study of stellar motions, and to the detection of stellar systems and stellar swarms.

The transition from the old to the new astronomy, from the period of precision in determining the positions of the heavenly bodies to the period of studying them with the spectroscope and applying photography, coincides with the period of Sir David Gill's activities in the world of science. That fact gives an enhanced value to the book under review and to the life of its author. The Description' takes a position such as Tycho Brahe's description of the Uranienburg Observatory, or Wilhelm Struve's of the Pulkova Observatory, held in their days; and for long it will be used as a guide to the design and construction of astronomical instruments of precision. The chief value of the History' for astronomers throughout the world is that it contains a condensed summary of the work of its author. Before Gill took up his duties in South Africa, Fallowes, Henderson, Maclear and Stone, with their assistants, had done the best they could as H.M. Astronomers at the Cape Observatory, controlled as it was by the Admiralty of an economising, and not too sympathetic Government; and the result was pitiable as the outcome of the premier Observatory of the southern hemisphere. From the moment when David Gill, the man of energy who always knew exactly what he wanted to accomplish, appeared upon the scene, this Observatory developed with giant strides, and became, before he left it, a model for all the world, fitted with the finest instruments of precision, and furnished with a devoted staff adding yearly to the published results which enriched astronomy, under the direction of their honoured chief.

David Gill, son of David Gill, J.P., of Blairythan, Aberdeenshire, was born on June 12, 1843. He became a student at Marischal College and University, Aberdeen. Thus he came under the genial influence of that lovable, unselfish and profound philosopher at Marischal College, Professor James Clerk Maxwell. He was one of the few to gain the full benefit from the lectures, for Maxwell (like Kelvin) was a failure in teaching the average young men in his class. After each lecture, he would carry on his instruction by conversation with his favourite pupils, and no one was more receptive than young Gill. At the age of twenty he went to Edinburgh

and made his first acquaintance with an astronomer, Prof. Piazzi Smyth, and an observatory, that on the Calton Hill. On his return he urged Prof. Thomson of King's College, Aberdeen, to assist him to give accurate time to the town of Aberdeen. His boyish enthusiasm awoke a sympathetic chord in the Professor. They unearthed an old transit instrument, cleaned and mounted it, and fitted up a clock to send electric signals to control other clocks indicating true Greenwich time. This gave Gill some training in the routine work of an observatory. Then the two set to work with a 3-inch telescope. This whetted Gill's appetite, and he bought, second-hand, a silver-on-glass speculum of 12 inches aperture. He designed and erected a mounting for the reflector, and with his own hands constructed the driving clock. With this fine instrument he observed stars, nebulæ and planets, besides photographing the moon (by the old process of wet-plate photography).

In 1870 he married Isobel, second daughter of Mr John Black, of Linhead, Aberdeenshire. They had no children, but their devoted affection, sympathy, and mutual helpfulness were evident to a wide circle of intimate friends. After his marriage he settled in the town of Aberdeen, near his observatory. His father, at an advanced age, retired from his old-established business, dealing in clocks of all kinds. He wanted his son to continue the business, and David, much against the grain, yielded to what he considered to be his duty. While he still spent his evenings on astronomy, he devoted his indomitable energy in the day to the work which lay before him, and he succeeded. He even set to work to perfect himself practically in the watchmaker's art, an experience which served him well in his subsequent dealings with delicate instruments. To the last day of his life there rested upon the mantelpiece of his study a beautiful clock, made with his own hands.

He was very successful in his efforts to obtain good photographs of the moon, at a time when this art was in its infancy. These photographs incidentally gave a new turn to his life. Lord Lindsay, who later succeeded his father as the Earl of Crawford, was considering the question of building an observatory at Dun Echt, not far from Aberdeen. Having seen these lunar photographs Vol. 221.-No. 440.

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he made Gill's acquaintance, saw his instruments, discussed astronomical problems with him, and yielded to the charm of his ardent enthusiasm for science, as many a young man has yielded in the last fifty years. Finally, these two men entered into a partnership for astronomical research, and Gill was appointed to the charge of the observatory, which they proceeded to set up at Dun Echt.

By this time David Gill's scientific character was completely formed, and the principles then accepted as his guide in all scientific work were never altered. They were explained, after his retirement from active observatory work, in his Presidential Address to the British Association in 1907. He speaks of learning the lesson

'that human knowledge in the slowly developing phenomena of sidereal astronomy must be content to progress by the accumulating labours of successive generations of men; that progress will be measured for generations yet to come more by the amount of honest, well-directed, and systematically discussed observation than by the most brilliant speculation; and that, in observation, concentrated systematic effort on a special thoughtfully selected problem will be of more avail than the most brilliant but disconnected work.

'By these means we shall learn more and more of the wonders that surround us, and recognise our limitations when measurement and facts fail us.' (Report, p. 25.)

It is not often that an astronomer has the opportunity twice in a lifetime, as Gill had, practically to create, equip and use a magnificent observatory in accordance with the highest ideals. Dun Echt Observatory (which with instruments and library were transferred to Blackford Hill, Edinburgh, by the late Earl of Crawford, and presented to the nation), and the Cape of Good Hope Observatory, are substantial memorials to the memory of Sir David Gill. The large volumes, numbering about thirty, issued under his direction, including the Dun Echt Observatory publications, the Annals of the Cape Observatory, the Geodetic Survey of South Africa, and the Cape meridian observations, as well as his contributions to the Royal Astronomical Society and to astronomical literature generally, will remain for ever a permanent record of the energy, fixed