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one of these is perforated and fitted with a lens, to communicate the light from a lamp upon the pier to the inner illuminating reflector, and from thence to the spider-lines, or wires, at the eye-end. These pivots, which are thirty inches apart, rest upon Y's, also of bell-metal; but, whether wisely or not may be questioned, the latter are additionally hardened by the introduction of a thin slice of Brazil pebble; the bearing of which is to the whole length of the pivot-cylinder. A well-constructed sensitive riding-level is placed over the cones when required, and stands upon both pivots; its glass tube is supported by the middle, and not by the ends, and it is furnished with a cross level and screw-adjustments, with counterpoise weight, for levelling it at right angles to the main axis. The instrument is exactly on the plan of that which I had made for myself, and mounted at Bedford, as described in my Celestial Cycle, vol. i. p. 329, from which it only differs in focal length, and a little in aperture. (See plate XI.)
Meantime Mr. B. L. Vulliamy, of Pall Mall, had undertaken to make a clock for the transit room, and a better train of wheel-work cannot be turned out of hand; but the beat is certainly inferior to that of Hardy's escapement. The frame is a particularly strong one, and all the parts are screwed together. The wheels and pinions are cut in very high numbers, which renders the action of the wheel-teeth in the pinions extremely smooth ; and the combination is such, that it seems to have all possible means to overcome its own friction. The escapement is Graham's dead-beat, with steel pallets mounted in a brass frame: and it will be recollected that the preference for this form arises from the dead-beat being so true, that no variation in the clock-train is likely to have effect upon the time of oscillation of the pendulum, even though it may alter the extent of it on the arc of vibration. The pallets are portions of a ring, and the fittings of the arms that carry them are entirely formed by turning. There is an adjustment to the pallets to open and close them, so that the teeth of the wheel shall fall safe on the rest of the pallet, and no more. The pivot holes are all made of fine pan brass, mounted in a setting, and the end-shake determined by regulating screws. The pendulum is suspended upon an independent support, entirely detached from the clock, the sole connexion between which and the pendulum is through the medium of the crutch, and consists of two adjusting screws. The clock was brought to Hartwell, and singularly well fixed, on November 14th, 1832; and by the side of its solid mahogany case two upright carriages and plates arise, in order to support two large brass brackets and stands, on which the lamps are placed for illuminating the clock's face.
As I had used a pendulum contrived by Mr. Jones, of Charing Cross, and considered that it obviated certain faults in the usual stirrup-support of a mercurial cylinder, I suggested that Dr. Lee's clock should be fitted with one.
Mr. Vulliamy's assent was thus expressed to me, in a letter, dated June 27th, 1831:
“I am much flattered by the confidence Dr. Lee reposes in me, and will spare no pains to make such a clock as shall give him perfect satisfaction: at least as far as is in my power.
“ With regard to the pendulum, after what you have said, I will with much pleasure make it as you require; but I must claim from you the promise which you made me when I saw your clock at Bedford, of obtaining from Mr. Jones the mode of applying the brass case or covering to the steel rod.
“ You must excuse my making a positive promise as to when the clock shall be done. I depend a good deal upon others as well as upon myself, and with the best intentions I frequently am lamentably deceived as to the time in which I can execute a piece of work. In this case, I feel I am under an obligation to you as well as to Dr. Lee to get the clock done as soon as practicable, consistent with its being well done, and you may rely upon my so doing.
“ What chance is there of my father's fine clock being admitted into your Observatory ? I have an idea it would perform uncommonly well.”
The pendulum thus alluded to, is still in high favour with me after more than twenty years' acquaintance with it, and is therefore entitled to especial mention. The very simple and effective adaptation consists of a round steel rod, long enough to pass through the mercury, and nearly to the bottom of the inside of the cylinder containing it. The suspension-piece at the top of the rod is a bit of watch-spring, in the usual way: but the lower end of the rod has a screw on it about two inches long, the threads of which are each equal to
one-thirtieth of an inch. This screw works into an appropriate piece fixed to the metal bottom of the cylinder's interior, thereby allowing sufficient adjustment, according as we would compensate for mean or sidereal time: and the inside of this recipient is carefully and well coated with cement and gum lac, so that the quicksilver can never come in contact with the brass surface. The steel rod is covered loosely with a thin brass tube, which is pinned to its upper end, and passes below to the top of the cylinder, over which is a brass cover with a slit at its side, embracing a pin which projects from the cylinder's outer surface. The broad edge of the cap is engraved and divided into one hundred and twenty parts; and, as the pendulum-screw has thirty threads in the inch, it is evident that the unit of the scale by which the cylinder with its mercury may be raised or lowered, is the stoo of an inch. A steel point for marking the arc of vibration, is fixed on the bottom of the cylinder; and there are screws for the necessary adjustments.
As the excellence of the mercurial pendulum depends on the relative action of the steel rod and the quicksilver, it is evident in this instrument that they are in constant communication with each other, for full six inches of the rod is always immersed in the mercury; which is not the case in the usual construction, where they are never in contact. Moreover, both the rod and the mercury are covered with the same material, which gives them a fair chance of acting together; whereas in the stirrup-construction, the quicksilver is contained in a glass vessel - a bad conductor of temperature—while the rod is naked and exposed to the air. Hence the rod may undergo many changes of heat and cold that the mercury is defended from; and the additional hamper which the old construction requires, must occasion a greater drag, however difficult it may be to appreciate such an effect.
Another point in which I have been rather nice in a clock's performance is the weight, which is often sufficiently rude to wear the pallets and endanger the pendulum spring. I therefore directed one to be made in divers cylindrical pieces, like my own, so as to be capable of being adjusted to any weight between four pounds and nine pounds, within one quarter of an ounce.
pendulum vibrates two degrees on each side of the zero point of the arc, that marks as much weight probably as ought to be employed permanently : at the same time there is no objection to increasing the arc of vibration, by increasing the maintaining power to a certain extent. In the Hartwell clock I found that, with seven pounds one ounce, the pendulum vibrated one degree fifty-four minutes on each side of 0, a range to which it has been confined. *
Much of this will be sufficiently trite to the magnates of practical science; but that is not altogether the class to whom these remarks are addressed, and fortunately it was never intended that the world should be peopled only with giants. Leaving the leaders, therefore, to their experience, knowledge, and plenitude of power in the great observatories, we are here only bent upon shewing any tyro, how he may enjoy the glorious heavens without bewildering himself in severe mental and mechanical drudgery, and meet the smiles of Urania without apprehension of encountering her frowns. It is therefore quite in place here, to insist on the advantages which a private establishment of this nature obtains from deducing a due registry of time; and to point out that the keeping an accurate clock-rate, is one of the neatest works of a good observer. The machines are, to be sure, so admirably constructed now-a-days, that little irregularity in the march need be dreaded in an interval of two or three days : but such are the imperceptible affections of escapement and pendulum, train and its drivers, in impulse and momentum, that no really zealous stargazer ought to leave their errors so long unascertained. Where occasional absence or bad weather interpose and prevent a due examination, then the goodness of the clock must be relied upon; and it is on this account that every practical man should be provided with mechanism capable of such good performance as to aim at excellence. Constructive ingenuity has certainly advanced so near to perfection, humanly speaking, in this art, that a clock merely requires to be well treated and looked to, and it will render the most satisfactory results.
It may be as well to advise those who dabble with the weights, to stop the clock on such occasions; as the pendulum should not on any account be suffered to vibrate when not under the influence of the maintaining power.
The clock being fixed, and the transit instrument rigorously drawn into the meridian by reference to the pole and circum-polar stars, it became necessary to establish permanent points of reference, by which not only the meridian, but other adjustments also, might be tested from time to time; and, as distant objects cannot always be seen in the day, and are precluded at night, I determined upon strengthening Dr. Lee's means by two of the meridian marks, of which I had found one so useful at Bedford (See Astronomical Society's Memoirs, vol. iv. p. 518). To mount these with a propriety due to the site, we had them inserted into blocks of marble cut at Bedford, from drawings made by Mrs. Smyth. The north mark is a representation of the Temple of Janus, as given on a large-brass medal of Nero inscribed, PACE POPULO ROMANO TERRA MARIQUE PARTA JANUM CLUSIT. See my Descriptive Catalogue of a Cabinet of Roman Imperial Medals, printed at Bedford in 1834, page 43 ; for the coin there described, Number 50, is that from which the temple is copied :