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transmitted through apertures equal in diameter to the small sphere of the different substances submitted to experiment. "The result of several trials made at the commencement gave for

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the intensity of an Argand burner. The oxide of zinc was also tried, but besides wasting away rapidly, it proved inferior even to magnesia.

"Of these substances, and also of their compounds with one another, lime appearing to possess a decided superiority, my subsequent experiments were confined to it alone; and by a more perfect adjustment of the apparatus, by bringing the maximum heat, which is confined within narrow limits, exactly to the surface of the ball, and by using smaller balls than those employed in the early experiments, a very material increase of light has been obtained. The mean of ten experiments, made lately with every precaution, gives for the light emitted by lime, when exposed to this intense heat, eighty-three times the intensity of the brightest part of the flame of an Argand burner of the best construction and supplied with the finest oil. The lime from chalk, and such as is known at the London wharves by the name of flame lime, appears to be more brilliant than any that has been tried."

It thus appears that before he had thought of any of the earths as a means of light, he had been experimenting with various pryotechnical preparations and chemical stuffs as lights to be used with the reflector, and had been perplexed by the shape and unsteady character of the flames; in fact, had his attention particularly fixed on the very difficulty from which the use of an earth offered escape. Experiment, in short, had brought him to the point at which he was fully prepared to picture the lime ball in the focus of the reflector the

moment he should think of it—or have it suggested to him—as a source of light.

The use of the Argands by Colby and Kater in 1821, may have first directed his attention to this subject. I think it is certain that he was engaged in his experiment in the winter of 1823, and probable that the fact was one of the reasons why Colby selected him to be his right-hand man in his preparations for the Irish survey. The chief was anxious that this survey should be more perfect than any yet executed, and particularly desirous to obtain improved means of overcoming the difficulties of observation which, it was anticipated, would be encountered in Ireland, at once owing to the climate and the size of the triangles. These means Drummond seemed to be likely to furnish; a light he had ready to hand, and was working to fit it for use on the survey; the other thing most wanted was a heliostat-a means of continuously reflecting the sun's rays from one point to another. And this Drummond supplied in the course of 1824 and spring of 1825.

In the paper already cited, he gives a brief sketch of the history of the use of the sun's reflection as an aid in survey operations :—

"The reflection of the sun from a plane mirror, as affording a point of observation that might be seen at remote distances, was suggested and employed by Professor Gauss in 1822, while engaged with a trigonometrical measurement in Hanover; and the result of the first trials made at Inselberg and Hohenhagen rendered it highly probable that it might be applied with much advantage to this purpose.

"The principle was adopted in this country when Colonel Colby and Captain Kater were engaged, in 1822, in verifying General Roy's triangulation connecting the meridians of Paris and Greenwich. At their concluding station on Shooter's

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. Our 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 sun. 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

* Letter to Mrs Drummond.

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