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We can but briefly review the Observatory's history during the century and a quarter that has elapsed since Flamsteed's death. He was succeeded, as we have above hinted, by Halley, then in his sixtyfourth year. Halley found the Observatory empty; but Government liberally placed funds at his disposal for the purchase and maintenance of instruments.

He died in his eighty-seventh year, leaving behind

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him a mass of observations, which, however, were in so confused a state that no use has ever been made of them. Bradley was appointed his successor in 1742. He made a noble series of observations, extending over the twenty years during which he held office. The star observations were at a later period reduced by Bessel, and form the base of his celebrated work the "Fundamenta Astronomia," and the whole

of his observations are, at the present time, in process of re-reduction by some distinguished continental astronomers. The two great discoveries of aberration and nutation were made by him; the first in 1729, and the second in 1748. Bradley died in 1762. His successor was Dr. Bliss, who lived only till March, 1764. The office next devolved upon Dr. Maskelyne, who for forty-four years performed its duties with wonderful assiduity; scarcely ever leaving the Observatory except on some important scientific business, and making all the important and delicate observations himself, although he had the cooperation of a skilful assistant. He superintended the publication of the "Nautical Almanack," a work of indispensable use to seamen, of which he edited no less than forty-nine volumes. At his death he left four large folio volumes of printed observations as the result of the patient labour of his life. The celebrated French astronomer, Delambre, says in his éloge upon Maskelyne, that if, through some catastrophe, the whole materials of science should be lost except these volumes, they would suffice to reconstruct entirely the edifice of modern astronomy. Maskelyne died in 1811, leaving behind him an enviable reputation. He was succeeded by Mr. John Pond, who held office till the year 1835, when ill-health compelled him to resign; and he died in the following year. Pond was peculiarly skilful in the theory of astronomical instruments, and in the interpretation of the results afforded by them. The present Astronomer Royal, in one of his official reports, states that he regards him as the "principal improver of modern practical astronomy." He increased the staff of assistants from one to six, and entirely reorganised the instrumental equipment of the Observatory.

Immediately upon the resignation of Mr. Pond, the present "Royal Observator," George Biddell Airy, Esq., then Plumian Professor of Astronomy and Experimental Philosophy, and Director of the Observatory at Cambridge, was appointed to the vacant oflice. Under his presidency the Observatory has been gradually augmented and brought to its present complete and perfect condition: old instruments, very perfect in their way, but still behind modern requirements, have been laid aside, and new systems introduced; every improvement that modern science could supply and every appliance that modern mechanical skill could suggest, have been made subservient to the utilitarian principles of the Observatory under its present organisation. But we will say no more of this, for we will shortly ask the reader to accompany us in a walk through the Observatory, to see for himself

e Aberration is an apparent displacement of the heavenly bodies, arising from the motion of the earth combined with the velocity of light. Light, travelling at the rate of 183,000 miles in a second, occupies eight minutes in its passage from the sun to the earth; so that if we observe the sun at any instant, we do not see it in the place it occupies at that instant, but in the place it occupied eight minutes before. Nutation is an oscillatory motion of the earth's axis, due to the action of the moon upon the spheroidal figure of the earth.

and judge the nature and importance of the various observations and investigations performed within that seemingly mysterious establish

ment.

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But before doing this it will be well for us to endeavour to comprehend the nature of the duties that devolve upon the practical astronomer: this will the better enable us to understand the various instruments that will come under our notice; for, knowing what has to be done, we shall be prepared to appreciate the more readily how it is done. The Royal warrant imposes upon the astronomer the task of rectifying the tables of the motions of the heavens and the places of the fixed stars." Let us see what this means. We will take the second item first, and treat of the fixed stars, for they serve as the landmarks or milestones by which the courses of the planets and other heavenly bodies are laid down. We will presume that all our readers know that the fixed stars are so called because they remain permanently fixed with regard to one another; but that they appear to move round the earth from east to west, in consequence of the earth's rotation; moreover, that they all move (apparently) round a common centre, which is in the direction of the earth's axis of rotation, and which is approximately indicated in the heavens by a bright star, generally known as the Pole star.

Now in order to fix the position of any point upon any surface whatever, it is necessary to employ two measures or co-ordinates; for instance, if we would define the position of a post in a field we should measure its distance from two adjacent sides of the field; or if we wish to indicate a certain letter on this page, we should say it is so many lines from the top or bottom, and so many letters from the right or left-hand side. Just so the astronomer, whose duty is to record the positions of the stars, has to measure the distance between each star and two fixed or reference points; and the question is, what points is One is conveniently provided for him; it is the celestial pole the imaginary point or pivot round which the heavens appear to revolve. The distances of the stars from this point are reckoned in degrees of angular measurement from north to south, and the resulting measure is called the star's North Polar Distance. The other point of reference is what is technically called the first point of Aries. It is the point of intersection of the equator and the ecliptic, or that precise spot in the heavens which the sun occupies at the time of the vernal equinox; it was once indicated by a bright star in the constellation Aries; the precession of the equinoxes has long since carried this star away from the place it then occupied, but the point in the heavens, which is now determined by observations of the sun, still remains a zero point for measuring the other element of a star's position, called its Right Ascension. These two measurements on the celestial sphere are analogous to the latitude and longitude as measured on the globe of the earth; the latitude answering to the north polar distance, and the longitude, measured from Greenwich, to the right ascension. The

first point of Aries is therefore to the astronomer what Greenwich is to the geographer or navigator. So much, then, for the fixed stars; their right ascensions and polar distances once determined, remain for a long period unchanged, excepting by the small and known influences of precession, &c., which it would be foreign to our purpose here to discuss. The stars' places thus obtained and arranged in order of right ascension, form what is called a star-catalogue.

The places of the fixed stars being determined, the next consideration will be the motions of the wandering stars, or planets. These motions have all to be inferred from places determined in the same manner as the places of the fixed stars. The planets are observed, and their ever-changing right ascensions and polar distances determined, from day to day and year to year. Their apparent courses across the sphere of the heavens are thus deduced, and from these apparent paths their true orbital motions are found by calculation. These motions are represented by numerical tables, which are called solar, planetary, or lunar tables, according as they represent the motions of the sun, the planets, or the moon. Each planet has its own representative tables, and it is in the formation of these tables from the places observed by the practical astronomer, that the high mathematical achievements of the physical or gravitational astronomer. are called into play.

From these considerations we are led to the conclusion that the chief duty of the practical astronomer is the determination of right ascensions and polar distances of heavenly bodies-stellar, planetary, or cometary. Other matters of more or less importance-to which we shall refer in their proper places-from time to time claim his attention; but the most essential subjects of his observations are the fundamental data above alluded to; and the principal instruments and appliances of a working observatory are those by which these fundamental observations can be most accurately made.

We will now ask the reader to accompany us, mentally, on a walk through our National Observatory, and we will pay Greenwich Park a visit for that purpose. The admissions to the Observatory are strictly limited to such persons as are likely to be benefited by visiting it. Idling sight-seers are carefully excluded; but all whose pursuits or studies give them the least claim to attention are welcome, and to these every possible information is afforded.

A few objects arrest attention without the walls of the edifice.

Physical or gravitational astronomy is that branch of the science which treats of the causes of the motions of the heavenly bodies; the term gravitational takes its origin from the circumstance that all investigations and deductions are based upon the Newtonian law of universal gravitation, which teaches that every particle of matter in the universe attracts every other particle, with a force varying inversely as the square of their mutual distances, and directly as the mass of the attracting particle.

For instance, the 24-hour electric clock, supposed by the uninitiated to be kept going by the sun; the public barometer, with its indices showing the highest and lowest readings during the past few hours; the little windmill, like a child's toy, on the roof; and the high pole, with a light at the top, conjectured to be a beacon to show the longitude at sea! All these will come in for explanation in their proper places, as we pass through the establishment.

But there is one other external object to which it is possible that we may not again have occasion to allude; this is an iron plate fixed against the wall, with a number of brass plugs and pins projecting from it, with the inscriptions, "British Yard," "Two Feet," &c., over them. It will probably be asked, What has a yard-measure to do with astronomy? It has a great deal. One important branch of practical astronomy is the measurement of time, and time is the only natural standard this earth possesses: it is the only thing that is invariable.

Now, the British imperial standard yard, by law established, is a measure of length, bearing a certain definite proportion to the length of a pendulum which, at a given temperature and under other specified conditions, beats accurately seconds of mean solar time. This is the connection between astronomy and yard-measures. Any one who desires to secure an accurate yard-measure, may do so by carrying to Greenwich a rod about a yard long, and truly adjusting it by means of the appliance there exposed for the public benefit. He will find two plugs, the distance between which is exactly a yard when the temperature of the air is about 60°, and two pins for the support of the rod to be adjusted. The plugs are bevelled off a little on their insides, and

the points that are exactly a yard apart are marked. upon their upper surfaces by arrow-heads. If the rod will not go in as far as the arrowheads, it is too long; if it passes them loosely, it is too short. Our cut shows the arrangement, with a rod in position. Similar plugs are provided for shorter measures, down to three inches.

Upon gaining admittance to the building, a suite of irregular and low-pitched, though substantial, buildings first strikes our view. Into one of these we will enter. It is the Transit Circle room, and we find ourselves in the presence of the noble instrument itself. We will briefly describe the uses of this instrument for the benefit of the

uninitiated.

We have already shown that the principal duty of the practical astronomer is the determination of right ascensions and polar distances; but we have hitherto said nothing of the modern means of effecting these determinations. Right ascension, we have scen, is the distance of a heavenly body from an imaginary point-or more properly, a

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