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meteorological department. In a parliamentary sense, it is the general who wins all the battles.
Till the spring of 1826, Mr Drummond remained invalided with his relations in Edinburgh. He was too weak this winter for severe study; but he managed, notwithstanding, to effect a variety of improvements in his light apparatus, which he was bent on fitting for lighthouse use. In the spring he returned to London, where he was occupied during the summer and autumn in experiments for forming the new measuring bars with which it was intended to measure the base at Lough Foyle. Late in the autumn he again took the field. The station this year was Slieve Donard, in the County Down. Drummond joined the camp in October, and continued there till late in November. As in the previous year, the tempests were fearful, and must have grievously tried his already weakened constitution.
In the course of the summer he had greatly simplified his heliostat. In the instrument as first devised, the direction was determined by one telescope which pointed to the station of observation, while the motions of the mirror connected with it were determined by another telescope with which the attendant followed the sun. This heliostat was very effective, but somewhat trouble
It had been part of the original plan to give it a divided circle by which its direction could be fixed; but this was not done, so that in practice a theodolite had to be used in connection with it. But if the theodolite was to be used, the directing telescope might clearly be dispensed with. By an ingenious contrivance, Mr Drummond dispensed with both telescopes, and reduced the heliostat to a simple mirror connected with a stand by a ball-and-socket-joint, so as to admit of
being readily turned in any direction. The direction of the station of observation being marked by a line on the ground, a small flat brass ring was placed in the direction at about 20 or 30 feet from the heliostat, and the attendant had nothing to do but to move the mirror till the sun's rays fully illuminated the ring before him. At short intervals, as the illumination grew faint, he had to move the mirror slightly till the brilliancy was restored. At the station of observation the light, reflected through the ring, appeared like a star. To this simple form be reduced the heliostat in time for its being employed in the season of 1826.
The instrument, in this form, was most efficient. It was used with success at distances exceeding a hundred miles—from Precelly, in South Wales, to Kippure, in Wicklow ; from the Keeper, in Tipperary, to Cullagh, in Fermanagh. It was so easy of management that it could be put in position and adjusted, from a few calculated distances, by a common point-fixer using a measuring-tape and mason's level. This was done at Cnocanafrion, in Waterford, which was thus observed from Bartrigaum, in Kerry, a distance of more than ninety miles. The instrument was rendered portable by reducing the size of the mirror. Packed with the directing-ring in a leathern case, the point-fixer slung it over his shoulders, marched with it to the station, and there set it up screwed on to the top of a stick. Besides rendering low and obscure stations readily visible, it subserved another important purpose. It was a means of identifying stations—a matter of great importance in extensive surveys. The point-fixer and observer simultaneously noted and recorded the times of observation, and from the record, the station, if lost, could easily be recovered. The heliostat, in this shape, remained ever after in use on the Survey, and was found more and more useful with every season's experience. If the Drummond light was not often employed, it was because the heliostat rendered resort to it unnecessary.
We now come to the operations of 1827 and 1828, and the inventions connected with them. In these years Mr Drummond was engaged in measuring the celebrated base of Lough Foyle—the most accurately measured base in the world, as Sir John Herschel assures me, except perhaps that measured by Maclear at the Cape by means of the same measuring apparatus. The site of the Lough Foyle base is on the eastern border of Lough Foyle, its northern end being in the parish of Magilligan, in the county of Londonderry, and its south end in the parish of Tamlaght, in the same county. The line crosses the river Roe, which is about 450 feet broad. The length of the base was 34,028 5 feet.
The merit of this work turns on two distinct classes of devices. The first are those connected with the selfcompensating measuring bars which were employed in the measurement of the base; and the second are those connected with the process of measurement itself, and employed for testing and securing its accuracy as it went along
In using simple rods or chains in measuring a base, it was necessary to determine the length of the measure at some definite temperature, and to reduce the measurement made at any other temperature to its equivalent at the standard one. Thermometers placed in contact with various parts of the measure were carefully observed, the temperature recorded, and a reduction made according to the experimentally ascertained rate
of dilation or contraction of the substance of which the measure was composed. Of course, every care had to be used to prevent changes in the length other than those due to temperature, e.g., changes due to bending or twisting. Such care being taken, a great degree of accuracy was obtainable in measuring with simple rods or chains, through making the necessary corrections for variations of temperature as the process proceeded. The corrections that thus constantly fell to be made were, however, not a little precarious, and were, moreover, unqestionably most annoying
The bars with which the base at Lough Foyle was measured were so constructed as to be self-compensating for changes of temperature. Certain points near their extremities maintain the same distance from one another at all temperatures.
The fact that different metals expand under heat at different rates had previously been made use of in various ways to maintain the constancy of the distance between two points. Graham, the inventor of the mercurial pendulum, was the first to turn the fact to practical account; while by his experiments, published in the “ Philosophical Transactions” for 1715, he laid the foundation for the subsequent inquiries and improvements which led up to the measuring bars. dulum consisted of a steel rod, carrying a jar of mercury at its extremity. The upward expansion of the mercury counteracted the downward expansion of the rod, and kept the centre of oscillation always at the same distance from the centre of suspension. Mr John Harrison's gridiron pendulum was the next application of the fact, and seems to have been executed without the least knowledge of what Mr Graham had done before
him.* He secured the constancy of the distance between the centres of suspension and oscillation by such an arrangement of sets of steel and brass rods in the same plane, that the upward expansion of the one set was counteracted by the downward expansion of the other. After him, Mr Frotheringham, M. Deparcieux, and Mr Elliot, much about the same time (1738), independently constructed compensation pendulums, consisting of two bars, one of brass and the other of steel, fastened together by screws, with an arrangement of levers to raise or let down the pendulum bobs. In these the compensation was effected at the extremity of the bars, and not, as in the gridiron pendulum, at the centre. After this date, there were numerous beautiful contrivances introduced, for the purpose of eliminating the errors to which pendulums of this construction were found to be liable. We see, then, that a century of invention had been devoted to the subject of compensation for changes due to temperature, in combinations of steel bars and brass bars for pendulum purposes, before the subject was thought of in connection with geodesy.t
The French geodesists were the first to employ compound measuring bars to measure base lines. But in the French bars, the unequal expansion of metals was not used to secure the constant length of the measure. These compound bars, by an ingenious device, were made to measure and indicate their variations in length due to temperature, so that the use of the thermometer in connection with them was superseded. They regis
See a Paper contributed by Mr Short to the “ Philosophical Transactions” for 1752.
+ See Yolland's “ Account of the Measurement of the Lough Foyle Base,” p. 7. Lond. 1847. Longman and Co. 1 vol. 4to.