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Hill seven or eight days had elapsed, during which Hanger Hill Tower, though only ten miles distant, having remained completely obscured by the dense smoke of London, tin plates were attached to the signal post, so as to reflect the sun towards the station at stated times on a certain day.
“At the hours for which they had been calculated these plates became visible, and the observations were in consequence immediately and easily completed. In the subsequent operations of 1823, recourse was again had on two important occasions to the same method, and with equal success. I allude to Leith Hill, near Dorking, in Surrey, and Wrotham Hill, in Kent, stations which it was of the utmost consequence to observe from Berkhampstead Tower, near Hertford. efforts to effect these observations having for some time been rendered unavailing by the thick mist so frequently overhanging the bed of the Thames, a series of bright tin plates was put up on both stations. Each set, consisting of six or eight plates, was attached to a smooth flat board, placed vertically by the plumb line, and turning on a pivot; the respective inclinations of the plates with the face of the board being determined, so that they might have the positions required for reflecting in succession the sun's rays towards Berkhampstead Tower, when the surface of the board was turned at right angles to the line of direction. Although this method admitted but of rude execution, it fully answered the purpose for which it was employed; the plates became visible in succession at the appointed hours, the duration of each varying with the inequality of its surface, but being generally from ten to fifteen minutes ; they were seen nearly at the same hours for some days before and after that for which they were calculated.
“The distance to Leith Hill is forty-five miles, and the observations were in this way completed without the hill itself having been visible during the whole of our stay, which was nearly three weeks.”
The utility of employing the sun's reflection as a point of observation being established by the result of these experiments, the problem to which Drummond
addressed himself was to invent an instrument, simple in its construction and easy of management, which might be used on all occasions and at any station. Colby's plates were a temporary expedient, suited only for the particular place and time for which the angles of the plates were calculated. The same pole and set of plates answered only for a single station and a short time, owing to the rapid motion of the sun, or rather of the earth in its orbit, quickly throwing the reflections wide of the station of observation.
Drummond's solution of the problem was as simple as it was happy and ingenious. His heliostat consisted of a mirror connected with two telescopes; the one, forming the axis of the instrument, for looking towards the station of observation, the other for looking at the
The former telescope being turned on the station, and brought, along with the mirror, to a position of horizontality by means of screws and spirit-levels, the mirror was so connected with the latter telescope, that, as the telescope was turned to the sun, the mirror, moving with it, went into position to reflect the sun's rays to the station of observation. It was a problem of geometrical construction prettily solved. He had done many as difficult in the class of Professor Leslie ; but this one was of his own proposing, and its solution proved of the highest practical importance. He made improvements on the instrument afterwards, which we shall notice in the proper place. The heliostat and the apparatus for exhibiting the light in survey operations, are fully described in the paper in the “ Philosophical Transactions."
By these two inventions Drummond armed the Survey officers with the most powerful means of overcoming the difficulties of observation by day and night. The
light was not long till it made a sensation beyond the small circle of the corps ; in the scientific world its splendour and utility were at once acknowledged. “It is with a melancholy pleasure,” says Sir John Herschel,* “ that I recall the impression produced by the view of this magnificent spectacle as exhibited (previous to its trial in the field) in the vast armoury in the Tower, an apartment 300 feet long, placed at Mr Drummond's disposal for the occasion.
The common Argand burner and parabolic reflector of a British lighthouse were first exhibited, the room being darkened, and with considerable effect. Fresnel's superb lamp was next disclosed, at whose superior effect the other seemed to dwindle, and showed in a manner quite subordinate. But when the gas began to play, the lime being now brought to its full ignition and the screen suddenly removed, a glare shone forth, overpowering, and, as it were, annihilating both its predecessors, which appeared by its side, the one as a feeble gleam, which it required attention to see, the other like a mere plate of heated metal. A shout of triumph and admiration burst from all present. Prisms to analyse the rays, photometric contrivances to measure their intensity, and screens to cast shadows, were speedily in requisition, and the scene was one of extraordinary excitement.” It must have been a proud moment for the inventor when he witnessed this enthusiasm in the elite of London scientific society. But there was a prouder in reserve for himthe triumph of his light and heliostat over obstacles to observation, which for months had impeded the progress of the Survey.
The first step in the Irish Survey was a general reconnaissance ” of Ireland made by Colby and Drummond in the autumn of 1824. They traversed the country from north to south, fixed upon the mode of conducting the survey, and selected the stations for the great triangulation, as well as the most fitting place for measuring a base. Enough has been seen of Colby's manner of making a reconnaissance of a country, to enable the reader to judge what exposure and fatigue were suffered in this expedition.
* Letter to Mrs Drummond.
In the autumn of 1825 the triangulation commenced on the Divis mountain, near Belfast. Officers were sent into Cumberland and the Isle of Man to recover and erect marks on the old stations there as points of observation from Divis for the connection of the Irish and English Surveys. While they were on this duty, Drummond, with his heliostat, light apparatus, and a complete observatory of meteorological and other philosophical instruments, was encamped on Divis preparing the station for the great theodolite. On the 23d of August a conspicuous object was placed on the summit of Slieve Snaght, the highest hill of Innishowen, about 2100 feet above the sea. This was an important point in the triangulation which connects the north of Ireland with the western islands of Scotland, and it was necessary that it should be observed from Divis. The party of observers on Divis were in camp as early as the date of marking Slieve Snaght. They continued there till the 26th October without being able to make the necessary observation. The mountain continued enveloped in a haze so impenetrable as to render unavailing every effort made for the purpose. All this time it seems not to have occurred to put either the heliostat or light in use. But now, late as it was in the season, Colonel Colby determined that Drummond should ascend the mountain, and attempt to overcome the formidable obstacle to the completion of the observations by the aid of his apparatus. He did so, and with what success we may see in the narrative of General Larcom :-“Mr Drummond took the lamp and a small party of men to Slieve Snaght, and by calculation succeeded so well in directing the axis of the reflector to the instrument on Divis, that the light was seen, and its first appearance will long be remembered by those who witnessed it. The night was dark and cloudless, the mountain and the camp were covered with snow, and a cold wind made the duty of observing no enviable task. The light was to be exhibited at a given hour, and to guide the observer, one of the lamps formerly used, an Argand in a lighthouse reflector, was placed on the tower of Randalstown Church, which happened to be nearly in the line at fifteen miles. The time approached, and passed, and the observer had quitted the telescope, when the sentry cried, “The light !' and the light indeed burst into view, a steady blaze of surpassing splendour, which completely effaced the much nearer guiding beacon. It is needless to add that the observations were satisfactorily completed, the labours of a protracted season closed triumphantly for Drummond, and the Survey remained possessed of a new and useful power.”
This year's operations did grievous mischief to Mr Drummond's health. A residence on the top of Slieve Snaght at such a season of the year might have tried one more robust. He was borne up for the time by the excitement attending the first practical test of his inventions, only to suffer the more severely afterwards. With what joy he hailed the success of his operations on the lonely mountain we fortunately may see from a