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with the repeating table, but the drum and microscopes being capable of independent motion, in this respect differing from the theodolite of Ramsden, in which the microscopes are fixtures, or only moveable with the outer case, which can be turned round on the stand for the purpose of changing, as it is called, the arc, then in fact performing, though rudely, the functions of a repeating table, whilst the divided arc moves round with the telescope, being attached by strong conical radii to a vertical hollow cone, which turns round upon a solid axis. As the focal length of the telescope of this instrument is 2 feet, it evidently unites theoretically the advantages of the large circular instruments of Ramsden and of the repeating circles, providing at once for accuracy of observation by a powerful telescope, and for precision of reading, both by a divided circle of considerable magnitude, and by the use of the repeating principle; if, therefore, it has not fully realised the expectations of its able makers, the result may be considered a confirmation of the other objection urged against repeating instruments, namely, the difficulty of ensuring that perfect concentricity of motion should be at first attained and subsequently preserved, between the moving portions of the instrument and those of the table. Though it is almost impossible to give an adequate idea of this very beautiful instrument without a figure (see Vol. 3 Academy Course, Plate VIII.), this brief description was necessary in order to show that General Colby was fettered by no old prejudices, but endeavoured

to introduce into the Irish survey every supposed improvement which had been adopted by Foreign geodesists, and to give it the fairest possible trial, by applying to its development the unrivalled mechanical ingenuity of Troughton.

Another important consideration was the nature of marks to be used, as objects for observation, and this again called forth all the ingenuity of Lieut. Drummond. The usual objects were conical piles of turf or of stone, 15 or 16 feet high, or staves, formed of planks, 20 or 25 feet high, and sometimes 1' 6" wide, which, in peculiarly favourable states of the atmosphere, could be seen distinctly 90 to 95 miles, but long intervals of time would necessarily intervene, when no satisfactory observation could be effected of such distant objects. In the first section of the Memoir has been noticed, the ingenious contrivance of Major Colby, by which he reflected the sun's rays, on a definite line, from pieces of tin nailed on a staff, at angles corresponding with the varying elevations of the sun, and acting, therefore, for some days, as a true heliostat. This was probably the first application of the principle to geodetical operations, but just before the commencement of the Irish survey, Gauss had prepared two forms of heliostat, which were described by Baron Zach, and used in the Hanoverian survey, the account of which was published in 1820, and which as a survey deserves special notice, as the celestial observations for determining the amplitude of the arc were taken with Ramsden's sector, lent for the purpose. These

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heliostats were simple and effective instruments, but the reflecting surface was very small, not being intended for use at very long distances. It is by no means certain that Lieutenant Drummond was aware of Gauss's inventions before he had prepared his own, although he was certainly cognisant of them very soon afterwards. In his first heliostat, the direction was determined hy one telescope, which may be called the "pointer," and the motions of a mirror, connected with it by an arrangement of rods or thin bars, something similar to those of a pantagraph, were determined by a small telescope with which the attendant followed the sun, the mirror moving simultaneously with it; so that the rays, when the sun was observed through this telescope, were falling on the mirror in such a manner as to be reflected from it in the direction of the pointing telescope. This was a very effective instrument, but it was troublesome in practice, as it required constant attention, and the position of the attendant, when using the small telescope, was awkward. Lieutenant Drummond therefore replaced it by another much more simple, in which the mirror, from 10 to 12 inches square, was connected with a stand by a ball and socket joint, so as to be readily turned in any direction. The line was now marked out on the ground, which must have been done with any form of heliostat, when intended for reflection to distances of upwards of 100 miles, as no directing telescope suitable for such instruments could have seen objects so remote, and

a small flat brass ring being placed in the direction, at about 20 or 30 feet from the heliostat, the attendant had nothing to do but to move the mirror until the sun's rays illuminated fully the brass annular disc before him, when he was certain that the rays were proceeding in the required direction; and at short intervals of time, when he observed the face of the ring becoming faint, to restore it to its brilliancy by a slight move of the mirror. This simple and effective heliostat was used with great success, at distances exceeding 100 miles, such as from Precelly, in South Wales, to Kippure, in Wicklow; the Keeper, in Tipperary, to Culcagh, in Fermanagh, and was so easy of management that it was put in position, and adjusted at Cnocanafrion, by Mr. James Flanagan, (a very intelligent civil assistant, then attached to the triangulation as a point-fixer) from measured distances, set off on lines to near objects, with which I had furnished him from calculation, a measuring tape and a common mason's level having been all the implements required for an operation apparently so delicate. Cnocanafrion, in Waterford, was thus observed from Bartrigaum, in Kerry, a distance of more than 90 miles. The heliostat was made generally useful by reducing the size of the mirror to a circle of 4 or 5 inches in diameter, when the instrument packed with its directing ring in a leather case, slung over the shoulders of the point-fixer, became portable, and at any station could be screwed on to the top of a stick, not only rendering low and obscure stations readily visible,

but also identifying them, a matter of no small importance in extensive surveys, as the point-fixer and the observer took care simultaneously to note and record the time of observation. I have been gratified to observe that this mode of reflecting the sun's rays to a definite point, known to every schoolboy, has been recommended by Mr. Francis Galton, the African traveller, as a useful aid to shipwrecked sailors when struggling on the ocean in a boat, and unable, from its lowness in the water, to attract the attention of passing ships. Without doubt it would be the surest mode of drawing observation on the boat, and for such a purpose a bright piece of tin, or of any polished metal, might, as in General Colby's first contrivance, be used as the reflector.

In addition to the heliostat, Lieutenant Drummond proposed to apply the property of lime, and several other substances, of becoming intensely luminous at a very elevated temperature, the light of lime being especially pure and vivid. This property had been applied before in the laboratory of the chemist, but Lieutenant Drummond had the merit of applying it first to geodetical and lighthouse purposes. The preparatory experiments were carried on, as usual, in the apartments of Lieutenant Drummond, and were watched with the greatest interest by all of us, as well as by several men of science not connected with our profession, amongst whom was the amiable Dr. Prout. Lieutenant Drummond soon perfected his apparatus with the aid of Mr. Simms, the lime being placed in the focus of a parabolic mirror, and

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