Page images
PDF
EPUB

CHAPTER VIII.

SURVEY OPERATIONS, 1825–1828 ; SIMPLIFIED FORM OF

THE HELIOSTAT; THE MEASURING BARS ; THE BASE AT LOUGH FOYLE.

The period of invention and improvement which preceded the commencement of the Irish Survey had other results besides those which have been described. To procure apparatus which would secure rapidity and accuracy in the work of the great triangulation, was not the only object in view. It was designed that the Irish Survey should be a more general Survey than any that had yet been made; should embrace not only geodetical facts, but also facts meteorological and topographical. The Engineer officers were becoming good physicists as well as good surveyors, and, while discharging their more immediate duties, were in various ways promoting the interests of science. The topographical department grew in importance as the work of the Survey proceeded. It owed most in the course of its development to General Larcom, who possessed special qualifications for advancing this branch. He unites literary and antiquarian tastes with an aptitude at once for science and for the work of administration. The department which Drummond created and fostered was that of meteorology.

During the period of preparation scarcely an instrument existed which Drummond did not consider with

prove it.

a view to rendering it useful in the Survey ; and for him to consider an instrument was to attempt to im

He long tried to improve the barometer. His favourite construction was the syphon, and he 'made one with his own hands which performed remarkably well. In the course of his experiments, he devised a singular mode of bisecting a reflected image of the surface—a ghost, as he called it; but it did not help him to any practical result. He at length, says General Larcom, abandoned the subject from a conviction, to use his own words, “ that the errors to which the barometer was liable from causes beyond control, were greater than the quantities he had been dealing with.”

The elaborate collection of instruments which he brought together in the meteorological observatory on Divis—the first Irish station—is said to have presented a singular spectacle on the mountain top. He carefully observed and recorded them, till the observatory was destroyed by a calamitous storm. • The season on Divis," says Colonel Portlock, “ was indeed a noble one. The camp became a school not merely of geodetical but of meteorological science. And though the difficulty of moving the delicate instruments from hill to hill, and preserving them from injury amidst the mountain storms, obliged Colonel Colby to abandon the use of some of them in subsequent stations, enough were retained to add the skill of practice to the theoretical knowledge which had been acquired at that first and most instructive station.” It is characteristic of Colonel Portlock that he should say not one word of Mr Drummond in connection with this "noble season” and “most instructive station.” He gives the whole credit to "the chief, and has none for the projector and creator of the meteorological department. In a parliamentary sense, it is the general who wins all the battles.

* The Larcom Memoir, p. 6.

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.

some.

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

« PreviousContinue »