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varnished; the iron bars were browned like the barrels of fowling-pieces, lacquered, and then smoked; and the surface of lamp-black so. produced was gradually removed during successive experiments till the requisite effect was obtained. This being done, the spaces covered with lamp-black were carefully marked, and the iron bars cleaned and revarnished, the same portions being again covered by the lamp-black mixed up with varnish. After this process they were again subjected to experiment, and if any alteration was required, it was made in the same manner. The difficulty consisted in producing the proper surface for the iron in the first set of bars, the variation of the others being determined with comparative facility.
“ In conducting these experiments every precaution was taken to ensure satisfactory results. The place selected was an apartment in the Ordnance Map Office at the Tower under ground, surrounded by high walls, and remarkable for the uniformity of its temperature. On the stone floor of this room were placed stone piers to which microscopes were attached, and a wooden platform, supported on blocks at a distance from these piers, was laid down to prevent any derangement that might be occasioned by the observers incautiously stepping too near them.
“In order to heat the bars they were carried to an adjoining closet, and placed on two beams near the roof; ignited charcoal was then spread on the floor and underneath the bars ; sheets of iron were placed about three feet above it, and a blanket spread under the bars; the object of the two latter precautions being to intercept the direct radiation, diffuse the current of heated air, and prevent the temperature of the bars from rising too rapidly. The closet having slowly attained its maximum temperature, maintained it very steadily for several hours, and by means of the occasional addition of small quantities of charcoal, a temperature of 120° or 130° might be kept up any length of time without varying more than two or three degrees. These precautions were taken with a view to induce equality of temperature in the bars on which the correctness of the compensation depended ; and when thus heated the bars
were removed, placed with great care under the microscopes, and the variation in the length observed at given intervals.”
The rates of changing temperature being equalised so that the fraction
Increment or decrement of the brass
= a constant,
a knowledge of the positions of the compensated points was obtained from the known dimensions of the bars and the experimentally ascertained value of this fraction. In the MSS. referred to, Mr Drummond gives a summary of the experiments made on each bar for
regulating the rates of changing temperature, ascertaining the rates, and determining the compensated points. These experiments, nearly ninety in number, were made between January 18, 1827, and 24th April of the same year. The tables exhibiting the results give the lengths of the bars in their different states, viz., before being heated, when heated, and after cooling ; the temperatures at which they were examined ranging over the scale from 40° to 200°. “The approximate position of the compensated point," says Mr Drummond, " was, in the first instance, calculated from the mean expansions of brass and iron; but, being found to differ from this determination, it became necessary to alter its distance from the pivots. There being no adjustment for this purpose, a slip of silver was attached to the tongue, and a series of dots marked upon it, so as to extend on either side beyond the true but unknown compensation point. These dots were within *0527 inch of each other, and were brought under the microscope in quick succession by means of transverse screws; and when the position of the true compensated point was determined with respect to these dots, the plate
was removed, a silver pin inserted (in the tongue) at the requisite distance, and a dot marked upon it.” The experiments made by Mr Drummond for effecting the compensations in the compensation-microscopes were of the same nature, and the general results arrived at are given in the paper compiled from the MSS. already referred to. The adjustment of the compensations in the microscopes was effected in June 1827.*
The details have been preserved of many of the experiments summarised in the paper just cited, as well as of many others in which Mr Drummond had been engaged in the course of the year 1826, and which were carried on partly in his chambers at Furnival's Inn, and partly at the Ordnance Map Office. The experiments of 1826 are very interesting, and are obviously preparatory for those of 1827, by which the measuring bars and their microscopes were finally made and adjusted. They were entirely conducted by Mr Drummond. There were in all forty-one experiments on the heating and cooling of brass, steel, and iron rods; on the effect of the nature of the surface in heating and cooling bodies; and on the rates of heating and cooling of brass and iron solid cylinders. With these he was engaged during July, August, and September of 1826, the 28th of September being the date of the last; the date of the first is not recorded, but it was probably a day early in July. The range of temperatures was very considerable, freezing mixtures being employed in the cooling processes, and charcoal fires in the heating. Captain Yolland says, that when these experiments were made, but little was known on the subject. In his Account (Appendix No. III.) will be * Yolland's "Account,” p. 17. + See Note by Captain Yolland, App. to his “ Account," p. 1.
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.