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look upon his want of friends, and other such obstacles, as so many triumphs to be obtained and so many trophies to be won.” This is as forcible as it is frank, and (as the event proved) it was true. The aspirant proposed to give up a profession in which he was engaged. Apropos of this, Drummond proceeds :—“There are many instances of men embracing one profession and rising to eminence in another ; but I fear there is an essential difference between such characters and his, not so much in point of abilities, as in ambition, eagerness, and exertion.” It is clear he had been carefully considering the conditions of success in the legal profession, which many of “the briefless” only begin seriously to do after years of disappointment. Had he thrown himself into it, no doubt he would have laboured in it with untiring zeal. Such, however, was not to be his career.
In the autumn of 1819 he became acquainted with Colonel Colby, when that officer was in Edinburgh on his return after the summer operations of the Ordnance Survey in the Scottish Highlands. When Colby, in 1820, was appointed successor to Mudge as superintendent of the survey, Drummond received from him and gladly accepted an offer to take part in the work. It was an opportunity of combining scientific pursuits with the military service; and for the time he abandoned his intention of deserting the corps.
THE ORDNANCE SURVEY.
That the general reader may appreciate the services of Thomas Drummond in the sphere of duty which he now entered, it is necessary to explain briefly the principal operations in geodesy, and to glance at the history of the Ordnance Survey of Great Britain, and the methods which, up to his time, had been employed in it.
Geodesy, as a practical art, falls into two main divisions, in each of which there are peculiar difficulties to be surmounted: the first of these regards the measurement of base-lines; the second, the triangulation of the surface to be surveyed and measured.
A base-line is an initial measured line, whose length is assumed as the unit to which all other distances calculated in the survey are temporarily referred. The exact length of the base in yards, feet, and inches being known, these other distances admit at once of being reduced to yards, feet, and inches. error in measuring the base must enter into all of them. For convenience in measuring any large tract of country—in other words, in constructing a Trigonometrical Survey—it is necessary that the length of the base should be a considerable multiple of the standard unit of length, several thousand yards at least ; and for the accuracy of the survey, it is necessary that this length should be measured most exactly.
In measuring a base-line it is necessary to level the ground, to define the line, by marking in a permanent way its two extremities, and to allineate it-i.e., make it so straight as to allow of its being foreshortened to a point. These things being done, the line must be measured with some measure, or instrument of definite length. The measure should possess several qualities, to give any high degree of exactness in the measurement. It should be (1.) as large as is consistent with its being easily manageable ; (2.) of invariable length ; (3.) so formed as to allow of its being easily kept throughout its length in the line to be measured; and (4.) so formed as to allow of the exact juxtaposition of end to end of the duplicate measures employed, if such juxtaposition be not by some contrivance rendered unnecessary. It is obvious that the errors of measurement, supposing the measuring apparatus itself to be perfect, are apt to increase with the number of times that the measure must be applied to the base to go over its length; and that if the measure expands with heat and contracts with cold, or expands with moisture and contracts with dryness, like a rod of wood, the result of the measurement must be uncertain, and may be erroneous. If, again, the measure be flexible, like a long rod or chain, there must be difficulty in keeping it fully stretched, straight along its length, and wholly in the line; and errors corresponding to this difficulty must be entailed on the measurement.
The base being measured, the next set of operations, those of the triangulation, commence. Some object is fixed upon, which is considerably farther from either end of the base than the length of the base-line. Theodolites, with delicately graduated circles, capable of measuring angles to an extreme nicety, are then placed centrally over the dots which mark the extremities of that line, and their telescopes are directed to one another until, as it has been graphically said, “ they look down the throats of each other.” The telescopes being in this position, are clearly both of them directed along the base-line. Each of them being now turned round till it looks straight at the object which has been fixed upon, the instruments are clamped, and the angles through which the telescopes have been turned are read off on the graduated circles. The angles are thus ascertained, which lines, drawn to the object from the extremities of the base, make with the base-line. The object, in short, is made the summit of a triangle in which two angles and the length of the side between them are known. Its distance from either end of the base can thus be ascertained by computation, and made available as a new and larger base. “Thus,” says Sir John Herschel, in a paper in which this subject is handled with his usual lucidity,* “the survey may go on, throwing out new triangles on all sides, of larger and larger dimensions, till the whole surface of a kingdom or a continent becomes covered with a network of them, all whose angular points are precisely determined. The strides so taken, moderate at first, become gigantic at last ; steeples, towers, obelisks, mountain cairns, and snowy peaks, becoming in turn the stepping-stones for further progress, the distances being only limited by the range of distinct visibility through the haze of the atmosphere.” In mapping a country, after the net-work of great triangles has been thrown over it, the great spaces comprehended
by them are filled in by a system of smaller triangles, so as to carry the survey to any degree of minuteness that may be required.
It will have been seen, in regard to the first set of operations, that the chief requisite in measuring baselines is such a measure, bar, rod, or chain as shall have fewest of those qualities, and be least subject to those conditions in the use, which are the sources of errors of measurement; and in regard to the second set of operations, that, as the triangles increase in magnitude, the means of making distant points steadily and distinctly visible are next in practical importance to the excellence of the instruments for measuring angles. It will be seen hereafter that Mr Drummond made most important contributions to geodesical science in regard to each of these sets of operations.
A sufficiently distinct notion has now, it is hoped, been conveyed of the sort of operations in which the Ordnance Survey engineers were engaged—the measuring of base lines on the one hand, and the work of triangulation on the other. Let us now glance at the history of these operations in Britain up to the time when Drummond took a part in them.
That the British Survey is an Ordnance Survey harmonises with the fact, that the first survey conducted in Great Britain had its origin, not in the interests of science and peace,
but in the conditions of public safety and the purposes of war. It was begun shortly after the Rebellion of 1745, in a survey of the Highlands of Scotland, the want of an accurate knowledge of the country having been much felt by the king's troops. This survey was conducted by General Roy, with the help of a body of infantry, whose head-quarters were at Fort Augustus. Begun in the Highlands, it was ex