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The Craig Telescope

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The construction of a monster reflecting telescope by the Earl of Rosse constituted for a considerable period a prominent topic of interest and conversation in the scientific world. The patience and perseverance of the noble projector under every kind of discouragement, and the unwavering faith with which, at a large outlay to himself, he prosecuted the enterprise to a successful conclusion, secured to him the admiration and esteem of all who took an interest in the higher departments of science, while the discoveries that have since been made through its instrumentality have amply borne out his anticipations and rewarded his exertions. Its was necessary, however, that something further should be accomplished. To those not conversant with the subject, it may be necessary to state that a reflecting telescope on a large scale must always be a work of exceeding difficulty, and comparatively limited utility. The possibility of constructing an achromatic instrument of a power equal to Lord Rosse's, and through which the object looked at could be directly magnified (as with an opera-glass), has long been considered extremely doubtful; in fact, beyond the reach of mechanical and optical appliances. This desideratum is, however, now on the eve of being supplied.
    In the course of a recent ramble on Wandsworth-common our attention was attracted by a singular-looking structure, consisting of a plain tower with a long tube slung by its side, surrounded by a wooden-hoarding to keep off intruders. On making inquiries we learned that it was a new monster telescope on the achromatic principle in process of construction, under the superintendence of Mr. W. Gravatt, F.R.S., for the Rev. Mr. Craig, vicar of Leamington. Having obtained an introduction, we inspected the instrument, and ascertained some particulars respecting it which may not be uninteresting. The site, consisting of two acres, has been liberally presented by Earl Spencer in perpetuity, or so long as the telescope shall be maintained. The central tower, consisting of brick, is 64 feet in height, 15 feet in diameter, and weighs 220 tons. Every precaution has been taken in the construction of this building to prevent the slightest vibration; but, if any disappointment in this respect should arise (which, however, Mr. Gravatt does not anticipate), additional weight can be obtained by loading the several floors, and the most perfect steadiness will be thus insured. By the side of this sustaining tower hangs the telescope. The length of the main tube, which is shaped somewhat like a cigar, is 76 feet, but with an eyepiece at the narrow end, and a dewcap at the other, the total length in use will be 85 feet. The design of the dewcap is to prevent obscuration by the condensation of moisture, which takes place during the night, when the instrument is most in use. Its exterior is of bright metal, the interior is painted black. The focal distance will vary from 76 to 85 feet. The tube at its greatest circumference measures 13 feet, and this part is about 24 feet from the object glass. The determination of this point was the result of repeated experiments and minute and careful calculations. It was essential to the object in view that there should not be the slightest vibration in the instrument. Mr. Gravatt, reasoning from analogy, applied the principle of harmonic progression to the perfecting of an instrument for extending the range of vision, and thus aiding astronomic research. By his improvements the vibration at one end of the tube is neutralized by that at the other, and the result is that the utmost steadiness and precision is attained. The ironwork of the tube was manufactured by Messrs. Rennie, under the direction of Mr. Gravatt. The object-glasses are also of English construction, and throw a curious light upon the manner in which an enlightened commercial policy has reaction upon and promoted the advancement of science.  Up to a recent period the flint glass for achromatic telescopes was entirely of foreign manufacture. Since the reduction in the duty great improvements have been made in this department. The making of the large flint glass was instructed to Mr. Chance of Birmingham, who at first hesitated to manufacture one larger than nine inches in diameter. Upon being urged, however, by Mr. Craig, he has succeeded in producing one 24 inches; perfectly clear, and homogenous in structure. Besides this, there is a second of plate-glass of the the same dimensions, cast by the Thames Plate Glass Company, either of which the observer may use at his option.  The manner in which these object-glasses are fitted into the tube is a marvel of artistic invention. By means of 12 screws, numbered according to the hours of the day, they can be set in an instant to any angle the observer may require, by his merely calling out the number of the screw to be touched. The object-glasses also move round in grooves to wherever it may be considered that a more distinct view can be gained. The tube rests upon a light wooden framework, with iron wheels attached, and is fitted to a circular iron railway at a distance of 52 feet from the centre of the tower. The chain by which it is lowered is capable of sustaining a weight of 13 tons, though the weight of the tube is only three. Notwithstanding the immense size of the instrument, the machinery is such that is can move either in azimuth, or up to an altitude of 80 degrees, with as much ease and rapidity as an ordinary telescope, and from the nature of the mechanical arrangements, with far greater certainty as to results.  . . . The preparations for this really national work have been progressing for the last two years under the superintendence of Mr. Gravatt as engineer and mathematician, but it is only about three months since the superstructure at Wandsworth-common was commenced, and it is already near completion.

Times August 23, 1852