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for its observatory. On this occasion Wrotham Hill, in Kent, and Leith Hill, in Surrey, were to be observed from Little Berkhampstead in Hertfordshire. There was great difficulty experienced in making these observations; the dense mass of smoke and fog which constantly envelope the metropolis, long defying every effort to pierce it. The observations were at last effected by means of an apparatus contrived by Colby. In the summer, Vetch and Drummond had been out on a station hunt; the fact is mentioned in two letters, belonging to this year, which have been preserved, but in neither of them is it stated what district was explored. The trigonometrical season was cold and wet, and though it lasted till the 14th of November, little work was done. This was to be the last season, for many years, of the British survey. It was dropped, not to be resumed again till 1838.

*

The cause of the interruption was the survey of Ireland, which was resolved upon in June 1824. The object of the Legislature, in directing a new survey of Ireland to be made, appears from the Report of the Select Committee appointed in 1824, "to consider the best mode of apportioning more equally the local burthens collected in Ireland." The object was to obtain a survey sufficiently accurate to enable the valuators, acting under the superintendence of a separate department of the Government, to follow the surveyors, and to apportion correctly the proper amount of the local burthens. These burthens had previously been appor

* His sister had by this time become a great invalid. In a letter, of date 20th November 1823, Drummond expresses joy at good news of her. "I shall look out for a pony chaise for her, and bring it down with me in the spring," a promise which he redeemed. All his letters show affectionate regard and anxiety for her.

tioned by Grand Jury assessments.

The assessments

had, in some districts of Ireland, been made by the civil division of plough-lands; in others by the division of town-lands; the divisions, in either case, contributing in proportion to their assumed areas, which bore no defined proportion to their actual contents. The result was great and much-complained-of inequality in levying the assessment, which it was a primary object of the survey to remove by accurately defining the divisions of the country. The Committee reported that it was expedient to give much greater despatch to this work than had occurred in the Trigonometrical Survey of England. They recommended that every facility, in the way of improved instruments, should be given to the Ordnance officers by whom the survey was to be conducted; and concluded with the hope, that the great national work which was projected "will be carried on with energy as well as with skill, and that it will, when completed, be creditable to the nation, and to the scientific acquirements of the age.'

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This survey was to task to the full Drummond's powers of invention. When the trial came he was well prepared for it. From the moment when he joined the service he had striven with all his powers to qualify himself for advancing its interests. He had thrown his heart into it, and eagerly cultivated all those branches of knowledge which bore on its necessities. He betook himself with renewed energy to the study of the higher mathematics. He became devoted to chemistry. For years he used to rise at four or five o'clock in the morning, light his own lamp and fire, and, taking a cup of coffee, study without interruption till eight or nine, when official duties claimed his attention. The days thus early begun were utilised

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throughout.

He had a special subject of study for every interval of business, so that no portion of his time passed unimproved.

He had attended the chemistry classes in the University of Edinburgh. In London he resumed the study under Professors Brande and Faraday, whose morning lectures at the Royal Institution he for some time sedulously attended. His friend Dr Prout, an enthusiastic chemist, is supposed to have given him a bias towards this study. Be that as it may, Drummond prosecuted it with zeal, always on the alert in this field, as in every other, to make the knowledge he acquired available to the service.

General Larcom states that the use of lime for the Drummond light had its origin in a suggestion received at the Royal Institution. "The incandescence of lime having been spoken of in one of the lectures," the idea struck him that it could be employed with advantage as a substitute for Argand lamps in the reflectors used in the Survey, for rendering visible the distant stations, because, in addition to greater intensity, it afforded the advantage of concentrating the light as nearly as possible into the focal point of the parabolic mirror, by which the whole light would be available for reflection in a pencil of parallel rays, whereas in the Argand lamp only the small portion of rays near the focus was so reflected. On this subject his first chemical experiments were performed. 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 with these simple means, and resolved that he would henceforth devote to his new pursuit the hour or two imme* Memoir, p. 5.

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diately after dinner, when, he said, he could do nothing else, remarking how much Dr Prout had done during the intervals of active professional occupations."

We have Drummond's own more full account in a paper "On the Means of Facilitating the Observation of Distant Objects in Geodetical Operations," published in the " Philosophical Transactions" of 1826. It is prefaced by a brief sketch of the history of the use of lights in survey operations.

"In the beginning of the Survey, General Roy, on several occasions, but especially in carrying his triangles across the Channel to the French coast, made use of Bengal and white lights prepared at the Royal Arsenal; for these, parabolic reflectors, similar to those with which our lighthouses are supplied, and illuminated by Argand burners, were afterwards substituted as more convenient, but they have been gradually discontinued, the advantages derived from them proving inadequate, from their want of power, to the trouble and expense incident to their employment. In the trigonometrical operations of 1821, carried on by Colonel Colby and Captain Kater, conjointly with MM. Arago and Mathieu, for connecting the meridians of Greenwich and Paris, an apparatus of a very different kind was employed for the first time-a large planoconvex lens, 0.76 metre square, being substituted for a parabolic reflector, and the illuminating body an Argand lamp with four concentric wicks. The lens was composed of a series of concentric rings, reduced in thickness, and cemented together at the edges. This apparatus resulted from an inquiry into the state of the French lighthouses, and was prepared under the direction of MM. Fresnel and Arago. Its construction and advantages are explained in a 'Mémoire sur un Nouveau Systême d'éclairage,' by M. Fresnel. The light which it gave is stated to possess 3 times the intensity of that given by the reflector. It was employed, during the operations alluded to, at Fairlight Down and Folkestone Hill, on the English coast; at Cape Blancez and Montlambert, on the French coast; the greatest distance at which it was observed being 48 miles, and

its appearance, I have understood from Colonel Colby, was very brilliant.

"But valuable as this apparatus may be when employed in a lighthouse, the purpose for which it was indeed invented and constructed, the properties of the simple parabolic reflector appeared to give it a preference for the service of the Trigonometrical Survey, provided a more powerful light could be substituted in its focus instead of the common Argand lamp.

"With this object in view, I at first endeavoured to make use of the more brilliant pyrotechnical preparations; then phosphorus burning in oxygen, with a contrivance to carry off the fumes of phosphoric acid, were tried; but the first attempts with these substances promising but little success, they were abandoned. The flames, besides being difficult and troublesome to regulate, were large and unsteady, little adapted to the nature of a reflecting figure, which should obviously, when used to the utmost advantage, be lighted by a luminous sphere, the size being regulated by the spread required to be given to the light. This form of the focal light, it was manifest, neither could be obtained nor preserved when combustion was the source of light; and it was chiefly this consideration. which then led me to attempt applying to the purpose in view the brilliant light emanating from several of the earths when exposed to a high temperature; and at length I had the satisfaction of having an apparatus completed, by which a light so intense was produced, that when placed in the focus of a reflector, the eye could with difficulty support its splendour even at the distance of forty feet, the contour of the reflector being lost in the brilliancy of the radiation.

"To obtain the requisite temperature, I had recourse to the known effect of a stream of oxygen directed through the flame of alcohol* as a source of heat free from danger, easily procured and regulated, and of great intensity.

"To ascertain the relative intensities of the different incandescent substances that might be employed, they were referred, by the method of shadows, to an Argand lamp of a common standard, the light from the brightest part of the flame being

Annals of Philosophy, vol. ii. p. 99.

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