Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/802

Rh 766 HEKSCHEL Astronomy, &c., and so went to sleep buried under his favourite authors ; and his first thoughts on waking were how to obtain instruments for viewing those objects himself of which he had been reading.&quot; It is not without signi ficance that we find him thus reading Smith s Harmonics ; to that study loyalty to his profession would impel him ; as a reward for his thoroughness this lei him to Smith s Optics ; and this, by a natural sequence, again led him to astronomy, for the purposes of which the chief optical instruments were devised. It was in this way that he was introduced to the writings of Ferguson and Keill, and subsequently to those of Lalande, whereby he was educating himself for an astronomer and for undying fame. In those days telescopes were very rare, very expensive, and not very efficient, for the Dollonds had not as yet perfected even their beautiful little achromatics of 2 inches aperture. So Herschel was obliged to content himself with hiring a small Gregorian reflector of about 2 inches aperture, which he had seen exposed for loan in a tradesman s shop. Not satisfied with this implement, he procured a small lens of about 18 feet focal length, and set his sister to work on a pasteboard tube of that length, so as to make him a tele scope. A tube of this construction naturally bent, and it was useless for all purposes but for the determined eyes of William Herschel. This material was soon displaced for tin, and thus a sorry sort of vision was obtained of Jupiter and Saturn and the moon. He then sought for a reflector of much larger dimensions from artists in London. No such instrument, however, was for sale; and the terms demanded for the construction of a reflecting telescope of 5 or 6 feet focal length he regarded as too exorbitant even for the gratification of such desires as his own. So he was driven to the only alternative that remained ; he must construct a large telescope for himself. His first step in this direction was to purchase the debris of an amateur s implements for grinding and polishing small mirrors ; and thus, by slow degrees, and by indomitable perseverance, he in 1774 had, as he says, the satisfaction of viewing the heavens with a Newtonian telescope of 6 feet focal length constructed by his own hands. But he was not a man to be con tented with viewing the heavens as a mere star-gazer ; on the contrary, he had from the very first conceived the gigantic project and the hope of surveying the entire heavens, and, if possible, of ascertaining the plan of their general structure on a settled and systematic mode of pro cedure, if only he could but provide himself with adequate instrumental means. With this view he, his brother, and his sister toiled for many years at the grinding and polishing of hundreds of specula, always retaining the best, and re casting the others, until the best of the previous perfor mances had been surpassed. This was the work of the daylight in those seasons of the year when the fashionable visitors of Bath had quitted the place, and had thus freed the family from professional duties. After 1 774 every avail able hour of the night was devoted to the long-hoped-for scrutiny of the skies. In those days no machinery had been invented for the construction of telescopic mirrors ; the man who had the hardihood to undertake the polishing doomed himself to walk leisurely and uniformly round an upright post for many hours, without removing his hands from the mirror, until his work was done. On these occasions Herschel received his food from the hands of his faithful sister. But his reward was nigh. In May 1780 his first two papers containing some of the results of his astronomical observations during the last six years were communicated to the Royal Society through the influential introduction of Dr Watson. Herschel had made the acquaintance of this excellent man and skilful physician in a characteristic manner. In order to obtain a sight of the moon the astronomer had taken his telescope into the street opposite his house ; the physician happen ing to pass at the time, and seeing his eye removed for a moment from the instrument, requested permission to take his place. The mutual courtesies and intelligent con versation which ensued soon ripened this casual acquaint ance into a solid and enduring regard. The subject of this first memoir was the varying lustre of several of the stars, and especially that of Mira in the constellation of Cetus. It had been long known to fade in brightness from nearly that of a star of the first magni- t ide down to invisibility in such telescopes as then existed. Herschel had examined it, and many other variable stars, for himself ; it was not, however, a simple or isolated phenomenon that engaged his attention ; but, regarding the stars as so many suns, he examined stellar phenomena as possibly leading him to some intelligent conception of what might be occurring in our own sun. The sun, he knew, rotated on its axis, and he knew that dark spots often exist on its photosphere ; the questions that he put to himself were Are there dark spots also on these variable stars ? do the stars also rotate on their axes ? or are they sometimes partially eclipsed by the intervention of some opaque and invisible bodies 1 And then he asked himself What are these singular spots upon the sun 1 and have they any practical relation to us, the inhabitants of this planet 1 To these questions he applied his telescopes and his thoughts ; and as light from time to time dawned upon his apprehension, he communicated the results to the Royal Society in no less than six memoirs, occupying very many pages in the Philosophical Transactions, and extending in date from 1780 to 1801, It was in the latter year that these remarkable papers culminated in the inquiry whether any relation could be traced in the recurrence of sun-spots, regarded as evidences of solar activity (allied to volcanic), and the varying seasons of our planet, as exhibited by the varying price of corn] HerscheTs solution of the question was scarcely final, and the question has recently cropped up again, with more than a renewal of its former interest. In the following year (1781) he communicated to the Royal Society the first of a series of papers containing the results of his telescopic inquiries in relation to the rotation of the planets and of their several satellites. The object which he had in view was not so much to ascertain the velocities or times of their rotation, as rather to discover whether those rotations are strictly uniform. From the result he expected to gather, by analogy, the probability of an alteration in the length of our own day. These inquiries occupy the greater part of seven memoirs extending from 1781 to 1797. In the course of these telescopic observa tions he lighted on the curious appearance of a white spot near to each of the poles of the planet Mars. On investi gating the inclination of its axis to the plane of its orbit, and finding that it closely resembled that of our earth, he concluded that its changes of climate also would resemble our own, and that these white patches were probably polar snow. Modern investigations have confirmed his conclusion. He also discovered that, as far as his observations extended, the times of the rotations of the various satellites round their axes are .in analogy with our own moon, viz., equal to the times of their revolution round their primaries. Here again we observe that his discoveries arise out of the complete and systematic and comprehensive nature of the investigation in which he is engaged. Nothing with such a man is accidental. In the same year (1781) Herschel made a discovery which, as we shall see, soon completely altered the character of his professional life. In the course of his systematic examination of the heavens with a view to the discovery of the plan of their construction, he lighted on an object which at first he supposed to be a comet, but which, by its subse-