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most delicate and difficult geodetical operation. To whichever be due the credit of originating this invention, the details of the contrivance and the execution of the project devolved on the latter, who, in accordance with what seems to have been a constant principle in his conduct, to leave nothing undone to ensure success, not content with entrusting, as many would have done, the adjustment of the compensation to an instrumentmaker, himself executed, in the midst of furnaces, ovens, and freezing mixtures, all the trials, manipulations, and measurements necessary to ensure success.

An anecdote may be mentioned here, which sets in a strong light this leading principle of hearty devotion to his object and its duties, to the exclusion not only of personal ease, but to the utter abnegation of all that egotistical feeling which induces so many to turn aside from suggestions of another with indifference, if not with aversion. While engaged in these operations the conversation between himself and a scientific friend happened to turn on the discovery of Mitscheclich, who had shown that in certain crystallised substances heat occasions expansion in some directions, and contraction in others, necessarily implying invariability in some intermediate directions. It was suggested, as a bare possibility, that such a condition might be satisfied by cutting mica in some certain direction to be ascertained by trial, in which case an inexpansive or naturally compensated measure would be obtained. Nevertheless, some time afterwards, the writer of these lines, happening to visit him in his apartments at the Tower, found him surrounded by strips of mica, and busied in working out the suggestion. The result, however, proved abortive in everything but the illustration of character it afforded."

It seemed at variance with the character here ascribed to Drummond by one who knew him well, that he alone “ of our little band” should have been indisposed to entertain the principle of compensation from giving the preference to an idea of his own. It appeared also that Portlock misunderstood the object of the mica experiments, which was to procure a naturally self-compensated measure ; and further, that these experiments, instead of having preceded those for constructing the compensation bars, were entered upon and abandoned in 1827, when the bars were in the course of being constructed. The basis of Portlock's case for Colby seemed to be thus seriously impugned. In this state of the facts it was felt that the merit of the invention should not be surrendered to Colby, except on evidence of a much more precise and satisfactory character being adduced.

Such evidence was after a time furnished by General Larcom. It appears that the bills of Troughton and Simms have been preserved in the Ordnance Survey Office, and show that the mica experiments belong to the spring and summer of 1826, and are of earlier date than any of the recorded experiments on the expansions and contractions of brass and iron bars and cylinders. It also appears that Mr Browning, who assisted Mr Drummond in the year 1826, is distinct in his recollection that the mica experiments were carried on in Furnival's Inn.* Larcom and Dawson are also clear in their recollections to the same effect, as will be

* The following entries occur in the bills of Troughton and

Simms :

“ 23 Mar. 1826. Apparatus for straining wire in mica experi

ments, £2. 1 June 1826. Brass mountings, adjustments, &c., applied

to apparatus employed in mica experiments, £1, 10s.”

66

seen from the subjoined extracts from a memorandum written by General Larcom :

My conviction with regard to the relative shares of Colby and Drummond in the design and execution of those instruments is that to Colby belongs the design, to Drummond the execution. Colby having himself used the previously existing English apparatus, and being familiar with the various instruments which had been used in the measurement of bases elsewhere, considered a new apparatus necessary. He resolved to adopt the compensation principle, and devised the form. He first satisfied himself the principle was sound, and tested the mechanical difficulties, which, he found, were all surmountable. He then devolved on Drummond the duty of superintending the construction, which Drummond, with the invaluable assistance of Troughton, successfully accomplished.

“The grounds on which I rest this statement of the relative shares of Colby and Drummond in the base apparatus are, personal knowledge and daily intercourse with all the parties concerned, having been myself one of the officers at the Tower at the time, and taking part in the early operations in the cold cellar and heated chamber, having been more than once at Troughton's with Colby, and often with Drummond in the evenings at Furnival's, where I also lived. No one at that time thought of Drummond as the inventor of the bars. He never claimed to be the inventor, and, I believe, would have been the first to repudiate the idea.

“ But that does not derogate from his merit. He made the bars, was the deviser and planner of the numerous and beautiful contrivances and experiments by which they were brought to perfection, and with his own hand executed most of the experiments.

I find among my letters from Colonel Dawson, in October 1840, when I was writing my own brief memoir of Drummond, the following paragraph :Drummond's indefatigable exertions in the construction of the bars, and in the measurement of the base in Ireland, you are yourself aware of. The principle on which the compensation depends was suggested by Colonel Colby, and the means by which it should be supplied ; but great credit is still due to Drummond for the ingenuity displayed by him in mastering many difficulties which were met with during the construction of the apparatus, and for the laborious experiments by which its perfection was at last established. Previous to the construction of the bars, Drummond had entertained the idea of using bands of talc for the purpose, and had made some experiments, which you may remember, at Furnival's Inn; and subsequently, on observing that the thermometers, when laid on the bars, do not immediately take and represent the actual temperature of the bars themselves, he suggested the use of thermometers instead of bars, to be made of a length suitable for the purpose. This idea, however, was, as you know, never worked out.'"

The evidence of Larcom and Dawson, who had

personal knowledge of the whole matter, must be accepted as conclusive. It is supported by the bills of Troughton and Simms, and by proof of the authorship of the bars having been claimed for Colby from the first. Captain Yolland, it now appears, had never heard of his claim being disputed. In 1828, in the “ Philosophical Transactions,” Captain Kater thus asserted Colby's right to the invention :-“The measurement of a base has hitherto not kept pace with the progress of other geodetical operations; but the elegant arrangement which LieutenantColonel Colby has recently imagined, for compensating expansions, and which already has been tried in Ireland with perfect success, leaves no doubt of the future accuracy of this most important part of trigonometrical operations.” It is satisfactory, moreover, that there is a witness-than whom there could be no better that Mr Drummond never claimed the merit of the invention himself. In this state of the facts the conclusion to be arrived at seems to be that the design was Colby's, and the execution Drummond's; that to the former belongs the general conception, and to the latter the realisation.

It should be remembered, however, that the execution of the design in this case involved something more than mechanical contrivance and delicacy of experiment. It required much ingenuity and inventiveness. The device for equalising in the two metals, by the use of varnishes, the times required for acquiring any change of temperature—without which there could be no compensation action—belongs to Drummond, as well as the contrivance of the silver plate, by means of which the position of the compensated points was exactly determined.

It is due to Sir John Herschel to say, that immediately on Colonel Portlock's statement being brought under his notice, he expressed himself satisfied that the merit of the invention belonged to Colonel Colby. He wrote: “I do not remember ever to have had any conversation with Drummond on the subject of how or with whom the invention originated. It was certainly from him, and not from Colonel Colby, that I first learned the principle of their construction; and, having no knowledge of any such bars having been constructed as those on whose adjustment and trial I knew him to be engaged, I took it for granted it was a contrivance of his own, and have always since spoken of them as * Drummond's compensation bars.'

So far as I see, Colonel Portlock's statement is perfectly plain and direct, and is not incompatible with any fact within my knowledge. In writing the passage at page 185 of my Lectures, I wrote in the belief and full impression of its correctness, never having heard it stated that Colby was the inventor. The book has now been in print for several months, so that it is too late to correct or in any way to modify that passage, which, were it otherwise, from what I now know, I should certainly do."

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