Page images
PDF
EPUB

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.

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 37 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 18 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

1

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

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

Lime,
Zirconia,
Magnesia,

37 times,
31
16

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

fact was

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

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

« PreviousContinue »