found the details of experiments conducted entirely by Mr Drummond in May 1827, on the two ten-feet iron standard bars belonging to the Survey. And in the same Appendix, No. IV., will be found the experiments conducted entirely by Mr Drummond in June and July 1827, for the comparison of two sets of the measuring bars about to be employed at Lough Foyle, with Troughton's five-feet brass scale floating on mercury, and of Troughton's scale with Shuckburgh's, and of these scales with themselves in different circumstances; also of the three-feet Shuckburgh with the parliamentary standard. The results of these fill tables occupying six pages quarto. Immediately before, and during, the measurement at Lough Foyle, Mr Drummond, assisted, for the first time, by Lieutenant Murphy, compared, by an extensive series of experiments, the whole of the measuring bars with one another and with the Ordnance standard. These comparisons were, after the completion of the work, continued at intervals by Drummond and Murphy down till the autumn of 1829. The compensation microscopes were, at or about the same times, tested and adjusted. The results of these tests and comparisons are exhibited in tables occupying about six pages quarto. Till the date of commencing the measurement Mr Drummond is represented by the authorative account as the sole experimenter on the measuring bars, which must, accordingly, be held to have been constructed by him. The bars were, no doubt, furnished in an approximate state of completeness by Troughton.

Thus far the history of the measuring bars is free from obscurity. Mr Drummond made them, whoever designed them. The origin of the invention, however, seemed for a time to be not a little uncertain. On the

one hand, Mr Drummond seems to have long been the reputed inventor. "For a long time," says Colonel Portlock, "Lieutenant Drummond was considered to be the inventor of the compensation bars."* And this early repute seemed to be confirmed by Sir John Herschel in his "Familiar Lectures on Scientific Subjects" (p. 187), published in 1866, in which the whole credit of the invention is ascribed to Mr Drummond. "The bars now actually used for base measurements," he says, are miracles of ingenious contrivance and delicate workmanship. They are self-compensating for changes of temperature; that is to say, the two fine dots which mark the two extremities of the measure remain exactly at the same distance from each other whatever be the temperature of the bars, which are compound ones of two differently expansible metals combined on a principle devised by the late Lieutenant Drummond." On the other hand, Colonel Portlock, General Larcom, Captain Kater, and others, appeared claiming the invention for Colonel Colby. An investigation of the facts was thus called for; and it seemed to be the more necessary, from the only published statement in support of Colby's claim being in several respects unsatisfactory. But for the nature of this statement, it might have been sufficient to accept, without farther inquiry, the verdict on the question of authorship agreed upon by Portlock, Larcom, and others, who had opportunities of being personally acquainted with the facts.

To support Colby's claim to this invention is a main point of Portlock's memoir of his life. To give force to what he has to say on the subject, he drops the indirect style, and speaks in the first person

"It is necessary that I should here speak in the first person, in order to convey graphically the first steps of Major Colby's invention. Every one who knew him in those days must remember how rapidly he moved or ran through the streets, rarely relapsing into a simple walk; and it was thus that I met him rapidly descending Tower Hill, when he took my arm, and, with the usual 'Come, my boy, I have something to talk to you about,' carried me with him to the Map Office in the Tower, which was not only the office for the business of the Survey, including the engraving of the maps, but also contained the private apartments allotted to Major Colby, as director of the work. When once there I was detained for the evening, and after dinner Major Colby fully explained to me the idea he had formed of a compensation measuring rod. [Here follows an elaborate account of the invention, not as a germ or conception, but as a thing fully and completely devised. The starting-point of the explanation is the gridiron pendulum, the principle of which is explained; and then follows Colby's plan :- In the mode proposed by Major Colby, two bars of different metals were to be fastened together in the centre,' &c.] Having listened attentively to Major Colby's explanation of the principle of his proposed compensation measuring rods, I felt satisfied that it would succeed in practice; but such was not the opinion of all the members of our little senate, as Lieutenant Drummond was, in the first instance, more disposed to go on with his own inquiries, and expected a better result from them than from the proposed bars of our chief."*

He proceeds to state that he accompanied Major Colby to Mr Troughton, who at once approved of the plan, and made a small three feet model bar. "The small three feet model bar was rapidly made, and from that moment Drummond became the most able and active assistant of Major Colby in conducting all the preliminary experiments." Elsewhere he more clearly defines the function which he assigns to Drummond in connection with the bars. "The laborious experiments," he says, "which * "Memoir of Colby," App. II. p. xi.

were made for comparing the standards with the recognised standards of measure, for determining the exact position of the compensated points of the measuring bars, and for examining various descriptions of varnishes in order to fix upon one which would equalise as much as possible, in the two metals, the times required for acquiring any change of temperature (a matter of the utmost importance, as upon that equality depends the perfection of the compensation action, whilst the rapidity of heating and cooling is very different in different bodies), were carried on by Lieutenant Drummond, assisted principally by Lieutenant Murphy, but occasionally by several of our little band, under the immediate eye of Major Colby, on the basement floor of the Ordnance Map Office. The result was everything that could be desired, and the base of the Irish Survey was measured under the immediate direction of the inventor of the bars in the summer of 1827 with the most perfect success, several portions of the measurement having been witnessed by some of the most eminent scientific men of the country." He takes no notice of Mr Drummond's experiments in 1826. As to the experiments from which Mr Drummond "expected a better result than from the proposed bars of our chief," we have the following account, introduced by the statement, that on the Irish Survey being resolved upon, Mr Drummond anxiously set himself to devising a new measuring apparatus.

"The quarters, in Furnival's Inn, of Lieutenant Drummond, became a laboratory and a workshop, and diligently did that highly gifted officer labour to produce a measuring apparatus which should be free from the alleged defects of those hitherto used. His last and favourite scheme was a riband, formed of slips of mica, and which it was supposed would be almost in

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variable in length, from its very low degree of dilatibility. As, however, this riband would have required to be supported, like the measuring rods of Svanberg, by a trussed plank, and to be kept straight by weights, it did not appear to ensure permanency of length, even had it been admitted as true that mica undergoes little perceptible extension by heat. During his trials, Lieutenant Drummond suspended the riband of mica by iron wires, but this system would not have answered in practice, as the expansion of the wires by heat would have affected their rigidity, and rendered different weights necessary for retaining them in one constant state of curvature. Encouraging his officers, as Major Colby always did, to pursue these inquiries, and taking the greatest interest in the results of their efforts, he did not himself remain idle; and though from his apparently careless manner they scarcely looked upon him as a rival, the master soon proved that in the race of invention he was able to beat the most ingenious of his 'boys,' as he familiarly called them."

Several of these statements appear to be incorrect. In Captain Yolland's account of the measurement of the Lough Foyle base, which was published under the direction of Colby, we have the work of each of those engaged in the operations for the measurement assigned to its performer on the evidence of official documents. The invention of the bars is indirectly ascribed to Colby, by a reference in the preface to the reasons by which he was induced to devise a new measuring apparatus. The earliest recorded experiments, however, are those of Drummond in his chambers at Furnival's Inn and the Map Office in 1826, in regard to the rates of heating and cooling of bars of brass, steel, and iron, and the effects, on these rates, of varying the surfaces of the bars. Neither Colby, Murphy, nor any one "of our little band," has any place assigned to him in connection with these experiments or those for equalising the rates of changing temperature in the bars, for ascertaining the