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the present instrument combines the offices of both the transit and the circle; giving one observer the power of performing the work of two,

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and obviating many inconveniences that the use of the separate instruments involved.

A glance at the engraving will now doubtless make this instrument

intelligible. Attached to the telescope is the circle which answers to the mural circle; around its circumference is a narrow band of silver, upon which are engraved the divisions representing degrees of angular measurement, of which the whole circle contains 360. These degrees are further subdivided into smaller intervals of five minutes, and the intermediate minutes and seconds, and decimals of a second, are what is technically termed "read off" by means of micrometers, six of which are used, and their mean taken, to eliminate errors of observation, &c. These micrometers are fixed to the pier seen on the right-hand side of the engraving, which is perforated to allow the divisions to be seen through it. The other circle attached to the telescope is a clamping circle, for the purpose of fixing the instrument rigidly during an observation. To particularise all the other details of the picture would far exceed our limits: they consist of counterpoises to various parts, apparatus for raising the instrument, and other appliances necessary for purposes of adjustment.

In front of the instrument stands the Transit Clock, already noticed as its indispensable accessory. Our first impulse on looking at this, the standard clock of the Observatory, and in perfect accord with the stars, is to take out our watches to see how much they err from true Greenwich time; but we are chagrined to find that they differ by a number of odd hours and minutes. The truth is, this is what is called a sidereal clock, and the time it keeps is sidereal, or star time. O hours, or noon, by it is the time the first point of Aries is on the meridian, and has no connection whatever with the civilian's noon, or 0 hours of solar time. A sidereal day is 4 minutes shorter than a solar day, consequently a sidereal clock never indicates the same time, relatively to solar time, two days together; but by a simple computation the astronomer can in a few seconds convert sidereal into solar time, or vice versa.

We have been so far precise in our description of this instrument, because it is the fundamental one of the Observatory, as it is indeed of every similar establishment. We will now leave it for a while, to return to it shortly, and ourselves "take an observation." But before we leave this apartment we must cast a reverential glance at an interesting relic here preserved: it is the identical instrument (represented at fig. 3 of the engraving of ancient instruments) with which Bradley made his immortal discovery of the aberration of light.

The next important instrument is the Altitude and Azimuth, or, as it is shortly termned, the "Altazimuth," to inspect which we must mount to one of the mysterious domes that surmount several of the Observatory buildings. This instrument was erected in the year 1847, for the sole purpose of observing the moon. Next to the sun, the most important of the heavenly bodies is the moon; for, independently of her use in regulating the division of the year into months, and creating the tides of the ocean, she is indispensable to nautical science, as her motions afford the only means of accurately determining the longitude Hence it is that the astronomers of Greenwich have ever paid unflagging attention to lunar observations. But, from the peculiar

at sea.

N. S. 1866, VOL. I.

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position of the earth's satellite in the celestial machine, her motions are of the most intricate and complicated order, and require the highest applications of physical science to resolve them. In consequence, she requires to be constantly observed in all possible positions in her orbit, and under all circumstances. When, however, she is near con

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junction with the sun, or about four days before and after "new moon," she cannot be observed with the transit circle, because she passes the meridian in full sunlight; and even at other times, when she would be well observable, the observation is often necessarily lost through cloudy weather. It therefore became necessary to devise some instrument by which she could be observed off the meridian but which should yet yield observations as accurate and trustworthy as those made with the

transit circle. For this purpose the Altazimuth was constructed. It consists of a telescope turning upon a horizontal axis supported by a pair of upright pillars, or "cheeks," connected at their upper and lower ends, and turning upon vertical pivots; the lower of which is supported upon a pier of masonry, and the upper by a triangular framework of iron. A horizontal circle, with a graduated silver band upon its edge, is firmly fixed to the stone pier, and is "read off" by four micrometers attached to the upright cheeks. This measures the azimuth of the object, or its angular distance, measured horizontally, from the meridian. A vertical circle is firmly attached to the telescope, and read by four micrometers fixed to one of the upright cheeks; this measures the altitude, or angular distance of the object above the horizon. The resulting altitudes and azimuths are afterwards converted into right ascensions and polar distances by means of trigonometrical calculation. A clock, similar to the transit clock, stands by the instrument, by which the time of transit over cobweb wires in the telescope is noted, in order to give the exact time at which the object observed occupied the place indicated by the readings of the circles.

· Since the introduction of this instrument, the number of observations of the moon formerly made at the Observatory in the course of each year, has been about doubled; and as a natural consequence, the value of the Greenwich lunar observations has been largely increased. It may amuse our monetary readers to know that a complete observation of the moon is assessed at a money value of about 107.; and the unfortunate observer through whose negligence or carelessness an observation is lost, is regarded somewhat as suspiciously as a clerk or accountant who delivers up his accounts minus that sum unaccounted for. Such losses are, however, extremely rare; only those observers being trusted with observations of the moon who, from long experience, have learned the importance of the trust imposed upon them. Some idea of the importance of the Greenwich lunar observations may be inferred from the circumstance that during the century ending with the year 1851, Greenwich contributed nearly 12,000 observations of the moon towards the improvement and perfection of the vexatious lunar theory; all reduced under the direction of the present Astronomer Royal, and rendered immediately available for the investigations of the physical astronomer; the lunar tables now in use being chiefly based upon these observations.

Doubtless the reader has wondered how the Greenwich observations of the moon are connected with navigation. A few words will serve as a reply. The observing astronomer observes accurately the position of the moon in the heavens at all times and under all circumstances. He turns his observations over to the physical astronomer. The physical astronomer deduces from them the laws that govern the moon's motions, and represents those motions by numerical tables. These tables are put into the hands of the computer of the "Nautical Almanac," who, by their aid, predicts the place the moon will occupy, with reference to proximate stars and otherwise, at every hour of the day and night

throughout the year, and publishes these "lunar distances" in that work, three or four years in advance, for the benefit of seamen starting on long voyages. The mariner observes the moon and stars near her with his sextant; and from comparison of his observations with the positions given in the "Nautical Almanac," computes his longitude, and ascertains the place of his vessel on the trackless ocean. Is not this a satisfactory reply to the query, cui bono, applied to the astronomer's labours? and ought it not to enhance our respect for our satellite, and promote her beyond the theme of poetasters' effusions?

We will now ascend to the interior of the very stupendous dome, or rather drum, that caps the south-eastern extremity of the Observatory buildings. In it is a magnificent specimen of the class of instrument known as the Equatoreal. It frequently happens that comets and small planets require-like the moon, and for the same reason to be observed off the meridian; and it is, moreover, necessary to have a good telescope mounted conveniently for gazing purposes, micrometric measurements of the discs of planets and distances between double stars, observations of eclipses, occultations, and twenty other different phenomena. All these are effected by means of the Equatoreal instrument, which consists of a telescope turning upon an axis supported by a framework or shaft placed parallel to the earth's axis, or inclined to the horizon at an angle equal to the latitude of the Observatory. This framework or shaft turns freely upon pivots at each extremity; and from the circumstance that it always lies in the direction of the poles of the earth, it is called the polar axis. (The polar axis of an equatoreal for an observatory at the equator would be horizontal; and vertical for one at either of the earth's poles, supposing one ever to be required there.) Graduated circles are affixed to the telescope, for observing polar distances; and to the polar axis for observing right ascensions. It will easily be imagined from a glance at the mechanical construction of this instrument, with its necessarily oblique-lying axis, that it cannot be nearly so stable, and therefore not nearly so accurate in its results, as an instrument which is subject to no distressing strains-such as the transit circle or altazimuth; and it is on this account only used as a secondary resource for determining places of celestial bodies. It is with this instrument that the use of the stars as "milestones" is involved. So thickly are the heavens studded with stars, whose absolute positions are accurately known, that when an object has to be observed "off the meridian" its distance is measured by means of the equatoreal from some star close to it; the position of the star being known, that of the object is thus easily determined.

But for many of the uses of this instrument, such as those we have above alluded to, great instrumental firmness is not required; but another appliance is requisite that makes it one of peculiar interest to unpractical or gazing astronomers. We dare say most of our readers have, at some time or other, looked at celestial objects through a telescope; perhaps, at least, some of them have invested a penny with the itinerant star-gazer who shows the wonders of the heavens from

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