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The theory of this meridian appliance was, as I have elsewhere mentioned, suggested to me by Baron de Zach, when we were journeying together from Genoa to Bologna, there to catch the solar eclipse of September 1820 annularly. The south mark is exactly the same as the north one,- but it is mounted on a miniature of the facade of the Temple of Concord at Girgenti, with its central columns omitted, for the insertion of the meridian plate :
In order to suffer as little alteration as possible from corpuscular or other action, these marble temples were placed on stout basement stones upon a solid brick-work foundation, which last was, for the same reason, carried no higher than was absolutely necessary. They were placed respectively at the distance of one hundred feet north and south of the observatory slit; and the reference was by means of two lenses, one in each window-sill, ground to one hundred feet focus, and mounted in brass frames, with tubes through the walls. The marks could of course be readily shewn, at any hour of the night, by the hand-lamps and a simple tin reflector.
It happened that while these arrangements were in hand, Mr. Davies Gilbert, late President of the Royal Society, came on a visit to Bedford, and was persuaded to accompany me to Hartwell, and inspect the new observatory. On being shewn the transit instrument, and acquainted with the end proposed, he warmly approved of the intention : “for,” said he, “it may be very unexpectedly useful, as stars may be taken here when the sky shall be obscured at the regular observatories. And how curious it will be if a place in Kamschatka, or China, or other distant region, should have its longitude determined some day or other from Hartwell!” Since this remark was uttered, Dr. Lee's records have been ransacked on several occasions, for he is disposed to attend to every proper application which is made to him. But perhaps the nearest fulfilment of the Gilbert prediction was, when the worthy and hardworking Captain Philip Parker King, R.N. of New South Wales, who was a visitor at Hartwell in August 1849, found eight moon-culminating starscorresponding to those he had observed in Australia in the years 1845, 6, and 7,—which he had in vain endeavoured to obtain elsewhere.
In this recollection, I am not insisting on the merits of moon-culminating stars beyond values that are recognisable. They are by no means adapted for determinating small differences of longitude with precision; for which nothing under one hundred complete sets, namely, fifty of each limb of the moon, should be attempted. With a fixed transit instrument, the Greenwich
, clock-stars may be used as culminators, though they are not, as with the selected stars, equally above and below the moon; and in all cases of small arcs, a single journey with a batch of chronometers will yield the longitude better than one hundred transits of the moon, which would occupy perhaps a couple of years, besides being attended with great trouble of computation. The longitude from one lunar culmination, is not unfrequently twenty-five seconds of time wrong. But, with all the possible imperfections on their head, my reason for recommending the consecutive observations of them was on account of there having been, about that time, a kind of conventional adoption of them in the observatories of Europe.
Although Mr. Maclear—the present distinguished Astronomer-Royal of the Cape of Good Hope—and myself, got a good arc between Biggleswade and Bedford by their means, as shewn in the Memoirs of the Royal Astronomical Society, vol. iv. p. 564, still, on several computations, I found that the results for such small differences would inevitably be anomalous.
But this does not apply to larger arcs : there, while other methods might err greatly, or be seldom available, the moon-culminators are not affected with greater errors and uncertainties than in the small distances. Yet in all cases demanding implicit faith, a large number of well-taken observations of both limbs must be accumulated.
\ 3. THE EQUATORIAL TOWER.
Although contentment is a virtue much vaunted in morals, it has not acquired great esteem in the sciences : nor
nor was it more than three years after building his transit-room, and furnishing himself with a beautiful fivefoot telescope, that Dr. Lee compassed the enlargement of his astronomical means, by purchasing a splendid object-glass, of five inches and eight-tenths in diameter. On his making this acquisition, I was again consulted as to adding an equatorial tower to the meridian observatory. I had, to be sure, sundry scruples on the occasion, inasmuch as the site, though very excellent for a meridian instrument, was extremely inapplicable for one which would be expected to sweep around : and I moreover maintained that the five-foot
achromatic was fully equal to occultations, eclipses, and all the extra-meridional requirements of the place. However, after a reasonable time, I withdrew my opposition, and considered the conditions of the problem with attention. In the first place the new object-glass, which had nothing but its brass cell, was carried to Mr. Dollond, with a view to its being armed with eye-pieces, and mounted precisely in the manner of mine at Bedford. We then took steps for fitting the tower symmetrically to the observatory, so as to form, as it were, a part of the original plan : the success of this is shewn by the lines of the whole building, as they stand at present :
When the addition was thus carried in our synod, I called in the aid of my zealous friend Mr. Charles May, then of Ampthill, but now of the wellknown house of Ransome and May, of Ipswich, under whose admirable arrangement the undertaking was finished. Determined that the foundation should be equally as stable for its purpose as is that of the transit, we again called sifted Roman cement and the very best brick-work into requisition ; and the structure quickly assumed an important appearance. Besides the north and south piers for the supports of the polar-axis, a smaller one was run up in the centre, as there was a suggestion for using a smaller instrument there occasionally ; and the bottom part of the pier-chamber was well floored with brick laid in cement, by which means the damp liable to arise from the ground is interrupted. And the following is a section through the structure, in a north and south line :
This tower has an interior diameter of fifteen feet, between walls which are fourteen inches thick in brick-work, capped with a Portland-stone coping in sixteen stout blocks, for bearing the iron channel and curb for the rollers on which the hemispherical dome, rising ten feet higher, turns. By the coping projecting outwards, it forms at once a moulding and, by a groove on the upper side, a channel for the rain to pass off without running down the wall. A castiron ring, made also in sixteen pieces, rests upon and is partially imbedded in this capping; the joints are made true, so as practically to be like a single