A party of sixteen non-commissioned officers and men, afterwards increased to nineteen of all ranks, under sergeant James Steel, was detached on the 1st May with sufficient camp-houses, equipage, and stores, to carry out the remeasurement of the base line on Salisbury Plain, by means of the compensation bars invented by General Colby.[[45]] No man or officer on the survey had ever seen the apparatus in position before; and sergeant Steel, therefore, has the credit of acquiring a full knowledge of the adaptation and uses of the various instruments belonging to the apparatus, unassisted by the teaching of any practician. This he achieved by more than three months’ unwearied study of some manuscript records on the subject, and by closely observing the results of a series of experiments which he conducted.
During the first fortnight, the line, six miles and three-quarters in length as the crow flies, was three times measured with the chain, marked off, cleared of wood, furze, and other obstacles, and again roughly surveyed. The little wooden encampment of the detachment was by this time in excellent order; and, after three days’ tedious work in testing the apparatus by comparison with the standard bar, the first compensation bar in the remeasurement was laid at Beacon-hill. Owing to the steepness of the ground, and other causes, progress over the hill was both slow and wearisome; but having once mastered the descent, the operation throughout its length presented less difficulties than were at first encountered. From time to time the sergeant communicated to the ordnance map office at Southampton the obstacles, both physical and instrumental, he met with in his progress, and the contrivances he resorted to, to overcome them. The journal so sent was full of practical instruction, of a kind to be easily acquired on future reference, and was replete with interesting information.
The distribution of the party gave ample employment to every man, and the division of labour was adapted to the attainments of the men and the necessities of the duty. Corporal William Jenkins assisted the sergeant at the bars and microscopes. The latter compared the microscopes with the standard on Sundays; and frequently, after a severe day’s work, the same process was necessarily gone through, and other adjustments of the instruments effected. Corporal Edward Harkin constantly attended to the aligning instrument, whilst one man assisted him in preparing the stations, &c.; two privates levelled the triangles for the feet of the supporting stools for the bars; two attended to the adjustment of the stools on the triangles, levelled the camels on them, and moved forward the microscopes, &c.; two carried forward the bars and point-carriers, and levelled the former and fixed the latter; one registered the bars and microscopes, and otherwise aided in moving them forward and adjusting them; one, a carpenter, made the pickets, and repaired the mallets, tents, &c.; four attended to the shifting and placement of the tents; one was sentry over the bars at the dinner hour and during the night, to prevent any disturbance in the apparatus; and two attended to the domestic and miscellaneous duties of the huts.
The camp occupied three different positions on the line. It was thus moved twice forward. On each occasion, for a few days, no progress was made in the remeasurement, and sergeant Steel with two privates, filled up the interval in comparing the bars and microscopes with the standards. In the meantime, the remainder of the detachment fitted up the portable huts in the position selected for them.
Great nicety and precision were required in the placement of the bars; and so rigidly did the sergeant enforce the strictest exactness in their alignment and contiguity, that he would not order the “move forward” until he satisfied himself that the possibility of an error in the operation was not likely to exceed the 10,000th part of an inch. In this way the work was continued till the 16th October, 1848, when the 3,484th bar shot over the old Sarum terminus of the line. This was followed by a spontaneous cheer, hearty and sustained, from the assembled party who thus commemorated the successful accomplishment of the operation. By previous computations from the Lough Foyle base, the perfect accuracy of the remeasurement was proved; for, not only did the predetermined bar reach the gun, but the very inch of it entered the muzzle.
To ascertain by the usual computations whether any error by the omission in the registry of a bar or microscope could be detected, the line was divided into three parts, and each part was used as a base for a minor triangulation. Very great care was taken in executing this triangulation, but it failed to discover any inaccuracy in the measurement. Sergeant James Donelan and corporal William Jenkins, with the two 3-feet instruments, carried out this special service.
The results of the two measurements stand on record as under:—
| By General Mudge with Ramsden’s steel-chains in 1794 | 36575·64 | feet. |
| By sergeant Steel, with Colby’s compensation-bars, in 1849 | 36577·95 | ” |
| Computed from Lough Foyle base | 36577·34 | ” |
The precision of the two operations by such different instruments is strikingly close and beautiful, and not only illustrates the excellence of the instruments, but the perfection of the work.
On the completion of the service, corporal Jenkins was intrusted with one of the great theodolites, and removed with a camp party from the base detachment to a mountain station. The remainder were soon dispersed on the general duties of the survey, and sergeant Steel, after again comparing the bars and microscopes with the standard measures, returned with the compensation apparatus, &c., to Southampton.[[46]]