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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.”
The details of this investigation might perhaps, with propriety, have been decently buried out of sight, and the result merely laid before the reader. It seems, however, not undesirable that, the matter having been fairly sifted and settled, there should be a record of the evidence on which their respective shares in the work fall to be assigned to Colby and Drummond. And such a record is, perhaps, the more desirable, that in reliance on the passage in “The Familiar Lectures” the whole merit of the compensation bars has been recently, in a work of reference, assigned to Mr Drummond.”*
We come now to the devices employed for testing the accuracy of the measurement as it went along.
It would be out of place in a work which it is hoped may have interest for the general reader, to enter into the particulars of the contrivances for securing the levelness and straightness of the base-line. Everything that ingenuity could devise was done to attain these ends. A novel feature in the measurement, however, and one which falls to be noticed, was the employment of triangulation for the verification and extension of the base. This was a device of Drummond's to prevent the possibility of the measurement of a single bar or microscope being omitted in the daily record, as well as to ascertain the probable error which might be involved by adding a portion to the base on its own prolongation by triangulation. Remeasurements were also made whenever there was reason to apprehend that there had been any accidental derangement of the apparatus. Sir John Herschel and Mr Babbage were at one of these remeasurements, when the coincidence of the two measurements was so close as to be surprising
“ Chambers' Encyclopædia," Art. TRIANGULATION.
even to men accustomed to deal with the smallest quantities measurable by the micrometer. An admirable pencil sketch (copied in the annexed woodcut), taken by Sir John on the spot, and now in his possession, shows the process of measurement, the six bars in position under their protecting tents, and the officer in charge engaged in observing the compensation microscope. It is the sketch alluded to in the following letter—the only account of the measurement, written by Drummond, that has been preserved, and which has been put into my hands by Sir John Herschel, to whom it was addressed :
“ MANCHESTER, Nov. 25, 1833. “MY DEAR SIR,—In consequence of the wandering life I am at present leading, and of soine mistake at a country post-office, your lettter of the 28th ult. reached me only a few days ago. Not having a single paper or document of any kind to refer to, I must answer your questions about the Irish base-line rather from the impression which some previous inquiry has left upon my mind, than from any very distinct recollection of the circumstances which have produced it. The distance is about 74th miles, and the error I believe not to exceed 2 inches. I shall endeavour, to the best of my recollection, to state to you some of the grounds of my belief. The line is intersected by the river Roe, not deep, except for a few yards, but having a width of 480 feet. We looked forward to the crossing of this river with some degree of apprehension; it was necessary to drive piles the whole way across to support the bars, an operation of some difficulty and expense; and although every precaution was resorted to in order to render them steady, it was not sufficient to prevent the tremulous motion produced by the current. It was therefore considered indispensable that this portion should be measured twice. On the first occasion
* It appears from the official account that this visit was paid on the 13th September 1827, a few days after the measurement commenced.