During this period he also devoted considerable attention to the study of chemistry, and attended the lectures of professors Braude and Faraday. The society of his friend Dr. Prout, it is believed, first led his mind to this science, and, with his usual felicity of application, he soon made his new knowledge available to the duties ho was employed on. The incandescence of lime having been spoken of in one of the lectures, the idea struck him that it could be employed to advantage as a substitute for argaud lamps in the reflectors used ou the survey for rendering visible distant stations; because, in addition to greater intensity, it afforded the advantage of conceutrating the light as nearly as possible into the focal point of the parabolic mirror, by which tho whole light would be available for reflecting in a pencil of parallel rays, whereas of the argand lamp only the small portion of rays near the focus was so reflected. On this subject his first chemical experiments were formed. Captain Dawson recollects Drummond mentioning the idea when returning from the lecture, and that on the way he purchased a blowpipe, charcoal, &c. That evening he set to work, and resolved that he would thenceforth devote to his now pursuit the hour or two before his evening studies began, remarking " how much Dr. Prout had done during the intervals of active professional occupations." At this period (1824) a committee of the House of Commons recom mended that the survey of Ireland should be begun, and that Colonel Colby should make arrangements for carrying it on. The objects of this survey required a work very different from the survey in England. Except himself aud a small number of officers, everything was to be formed by Colonel Colby for the work. Instruments of improved construction were required. Among others, a means of rendering visible distant stations ICU desirable. The recent experience of the Western lelaude had shown the probability that in a climate ao misty as Ireland the difficulty of distant observations would be greatly Increased, and Colonel Colby at once saw the important results which might follow such an improvement of the lamp as that which Drummond bad devised. Under his judicious advice the experiments were prosecuted, and were rapidly attended with success: their progress and results are detailed by the author in the 'Philosophical Transactions' for 1826, as well as the first application of the lamp to actual use in Ireland. When a station, Slieve Snaught, in Donegal, had long in vain been looked for from Davis Mountain, near Belfast, the distance being sixty-six miles and passing across the haze of Lough Neagh, Mr. Drummond took the lamp and a small party to Slieve Snanght, and by calculation succeeded so well in directing the axis of the reflector to the instrument, 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 tho tower of Randalstown church, which hap pened to be nearly in the line at fifteen miles. The time approached and passed, and the observer bad quitted the telescope, when the sentry cried, "The light 1" and the light indeed burst into view, a steady blaze of surpassing splendour, which completely effaced the much nearer guiding beacon.
An apparatus invented by Colonel Colby, consisting of flat plates of polished tie, at angles calculated to reflect the sun's rays in the required direction, was resorted to, and with entire success. It was however somewhat elaborate ; because, from the rapid motion of the sun, or rather of the earth, in its orbit, the same pole and set of plates would only answer for a single station, and for a short time on a very few days; the principle however was now obvious, and the elements which had been called into play soon suggested the more perfect instrument nom a calculation, of which the variables were the relative position of two stations and that of the sun, a happy step led to an instrument by which the problem should be as it were solved by construction—a telescope in the line between the objects, connected with one to be directed on the sun and carrying a mirror: such accordingly was the first heliostat of Drummond ; its mechanism is described in the paper already referred to (` Phil. Trans.,' 1826), and,
like the lamp, it was used auccessfully in the first season of the trigonometrical operations in Ireland. It NM originally intended to give this instrument a divided circle, by which its direction could be fixed ; but as this was not effected, a theodolite was necessary in con junction with it, and practice soon showed that if a theodolite were used, a more simple and less costly heliostat might be adopted. This accordingly was devised by Mr. Drummond before the second season ; the direction being effected entirely by the theodolite, instead of being as before dependent partly on the theodolite and partly on the adjust ments of the heliostat. The telescope of the heliostat now became useless, and it remained a simple mirror moveable in two directions, that is, on a horizontal and on a vertical axis, of which the light was guided by a directing staff previously placed by aid of the theodolite. This instrument proved so satisfactory that it has aver since remained in the form adopted on the survey, and it is every season found more and more useful. By its aid several observations have been made at distances exceeding ono hundred miles ; and such is its facility of direction—owing also no doubt in some degree to the great divergence which even the beet mirrors give to reflected light—that the theodolite is now frequently dispensed with, and by a few simple distances and measurements computed beforehand, a single soldier is sent with a heliostat to some remote mountain or island, with tolerable certainty that his reflection will be seen as soon as the sun shines after he reaches it.
Mr. Drummond's original heliostat was not completed till 1825. A heliostat had also been invented by Professor Gauss, of Gottingen, in the process of a survey carried on by him in Hanover. A very simple one had also been used by the late Commander Mudge, of the Royal Navy, while eurveyiag on the coast of Africa in 1823-24, which con sisted merely of a sextant sent forward to the station to be observed, and so adjusted as to throw the aun'a light to the observer.
In the autumn of 1824 Colonel Colby made a general reconnaissance of Ireland for the purpose of fixing on the mode of survey, the choice of stations for the great triangulation, and the most fitting place for a base. Ile selected Mr. Drummond to accompany him on this tour. The Plain of Magilligan was chosen, and Colonel Colby's attention was next directed to a fitting apparatus for the measurement.
Colonel Colby's long experience had shown him the defects of the apparatus formerly employed, and he boldly devised one altogether new, in which compensating expansions were to be used to form an unalterable linear measure. The construction of the instruments required long and careful experiments, the charge of which was con fided to Mr. Drummond ; and so far as was necessary to prepare the instruments for use in the field, they were performed by him or under his direction. It occnrred to him that mica, which bad then recently been recommended by Sir David Brewster for pendulum rods, might be applicable to this now purpose. Colonel Colby allowed experi ments to be tried on that substance, but they were not satisfactory, and Mr. Drummond abandoned the idea. The apparatus was com pleted according to Colonel Colby's original plan, and successfully used in the base at Mag,illigan. In the measurement Mr. Drummond was again employed, and whenever Colonel Colby was absent on other duty the charge of the operations devolved upon him.