At the commencement of the survey, all promotion was suspended for a time, to enable Colonel Colby to select the ablest men for preferment. He found great difficulty in choosing individuals qualified for it; but in less than two years after, so satisfactory was the improvement made in the attainments and efficiency of the companies, that the Colonel felt it essential to create by authority, supernumerary appointments as a reward for past diligence and an incitement to future exertion. This measure was the more necessary, as the most important part of the work was performed by the non-commissioned officers, who were mostly detached in charge of small parties of the corps with an equal number of civil chainmen. Each non-commissioned officer was thus the chief executive of a certain portion of work, and was responsible for its correct and rapid execution to the officers of the divisions. On the 17th of January, the supernumerary appointments were sanctioned by the Duke of Wellington without limit as to number, and Colonel Colby made ample use of the reward. The advantage enjoyed by the supernumeraries extended only to pay, they receiving the rate of the rank to which they were appointed. Service in the supernumerary grades did not reckon for their benefit towards pension. From the 6th of September, 1827, to the 20th of November, 1828, with occasional intervals of cessation, a detachment varying from two sergeants and twenty-three rank and file, to two sergeants and six rank and file, were employed on the measurement of Lough Foyle base in the county of Londonderry. A strong detachment of the royal artillery was also employed on this service. The duties of the sappers did not extend to the scientific and more precise details of the operation, but were limited to those subsidiary services which were essential to the rigid execution of the former. Their attention, in fact, was confined to the labours of the camp, the placement of the triangular frames, pickets, trestles, and such other incidental services as were indispensable to obtain an exact level alignment for the application of the measuring bars. A non-commissioned officer invariably attended to the adjusting screws; another frequently registered the observations, another attended to the set of the rollers and the regulation of the plates; and a fourth, with a few men, erected the base tents, moved them forward to the succeeding series of bars, and looked to the security of the apparatus for the night.[254] All these duties, though of a subordinate nature, nevertheless required the exercise of intelligence, and much careful attention on the part of those employed. ----- Footnote 254: Yolland’s ‘Lough Foyle Base,’ p. 25-27.