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(the nearest for instance) of the scale could be brought, by means of the screw, accurately to coincide with 100 feet at the front end. That division, whatever it might be, was of course registered in the field book of the operation, together with the true temperature of the chain, as shewn by five thermometers, one being laid for that purpose in each coffer, and secured with white cloth from the sun's rays, as occasion might require.

Fifteen coffers were always arranged on the ground at the same time, comprehending a space of the base equal to the length of three chains, or 100 yards. The extremities of the first chain having been accurately transferred, in the manner above mentioned, to the brass scales on the tops of the central posts, and these remaining firm and motionless, as being wholly unconnected with any other parts of the apparatus, the chain was then moved forward into the second set of coffers, where the thermometers were also placed. In the mean time, the first set of coffers, now vacated, with their posts, &c. were carried on and arranged in the front, for the measurement of the second 100 feet: and so on continually with the others in succession.

Of the Survey of Romney Marsh previously to the Measurement of the

Base.

23. In the introduction to this Account, it has been mentioned that the Duke of Richmond had permitted Lieutenant Fiddes, of the Royal Engineers, to be employed in 1786 and 1787 in raising a plan of that part of the Marsh where, on examination, it should be found, that the base of verification might be the best executed. In justice to that Officer, I consider it as incumbent on me to say, that it was impossible for any person to fulfil the duties entrusted to him better than he did, either in the course of the survey, or subsequent measurement of the base, whereof he also had the

direction. The general instructions given to him were, that after having, by a base of his own, determined certain triangles in the neighbourhood of Dymchurch, Ruckinge, and Romney, by way of foundation for his work, he should preserve Ruckinge as the point whereon the alignement of the great base was to be directed, and vary the position of that end next the sea-wall in such a manner as to meet with the fewest local obstructions to the measurement between the two extremities. By inspection of the plan, Plate VII. which comprehends a tract of country of two miles in breadth, one on each side of the base line, it will be perceived, that besides the numberless ditches with which this singular plain is intersected, and which it was impossible to avoid crossing, there is almost in every field a watering pond for the cattle, many of them of considerable depth. Nevertheless, so very attentive had Mr. Fiddes been to the accuracy of his survey, that he was enabled, after several trials of other directions, to run a line from High Nook on Dymchurch Sea-wall, upon the small spire of Ruckinge Church, of the length of nearly six miles, without interfering with any one of the watering ponds, or meeting with any other local obstruction of consequence. So very minute was he in his remarks, and so accurate in the situation of particular trees, that in tracing his line with the telescope, he managed so as to avoid them all, a few insignificant bushes excepted; which I believe to be an instance of exactness scarcely to be equalled.

Pipes sunk in the Ground.

24. Permission having previously been obtained from the proprietors of the soil, pipes were sunk into the ground at the two extremities of the base, and also one on Allington Knoll, which last point with Lydd Church* form that side of one of the great

* Mr. Cobb, of Lydd, an ingenious gentleman, well acquainted with Romney Marsh, was so obliging as to present me with a manuscript map of that singular

triangles depending on the base on Hounslow Heath, to be first verified by the measurement of this new base. Every field is surrounded with a ditch, in cleaning of which the earth and mud are continually thrown out on each side, whereby flat dikes are gradually formed on either side. That the measurement might be carried on as nearly as possible in the same plane, that is to say, about fifteen or eighteen inches above the common surface, therefore narrow grooves were cut in these flat dikes, which the different farmers readily consented to without murmuring. Here it is to be observed, that there was no occasion for levelling the line, Romney Marsh having been formerly covered by the sea, and a considerable part of it, particularly towards the bottom of the range of hills that separate it from the Wealds of Kent, being still lower than the sea at high water, would again be overflowed by it, if much care and expence were not annually bestowed in securing and repairing the dikes, whereby it is protected. Thus the line of the base may be considered as an inclined plane, descending gradually about five feet from the mouth of High Nook pipe to within 246 yards of the Ruckinge end, where the ground in that direction seems to be the lowest. Thence it rises com

paratively suddenly, about fifteen feet, to the mouth of the pipe situated in a small field immediately adjoining to Ruckinge Churchyard.

Result of the Measurement.

25. Lieutenant Fiddes, in the course of his trigonometrical survey, and of the different measurements he had actually made of the line with a common iron chain, which from time to time was

plain, compiled by himself from actual surveys, where the names and boundaries of the waterings, and many other curious particulars, are very distinctly expressed.Our plan of the base has therefore derived advantage by adhering to such respectable authority.

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compared with standard rods of deal, had determined the total length of the base within a few feet of the truth, before the ultimate operation began. He had likewise driven into the ground, at the end of every thousand feet, a strong picket, which were numbered 1, 2, 3, &c. from the pipe at High Nook to the 28th near Ruckinge. In all this preparatory part of the business he had no other assistants than the artillery-men of his surveying party. But for the ultimate determination, the aid of some person in whom he could confide for the management of the operation in general, and particularly for the adjustment of the scale at one end of the chain, while he himself was adjusting that at the other was absolutely necessary; therefore Lieutenant Bryce, of the Royal Artillery (now of the corps of Royal Engineers), an attentive officer, and mathematician, was left with him for those essential purposes. These two gentlemen began the operation on the 15th of October, and after experiencing many difficulties, arising from the badness of the weather in that late season of the year, and the defectiveness of the apparatus, it was only by dint of great labour, and the utmost perseverance, that they were enabled to accomplish the measurement on the 4th of December following.

The annexed table of the base, which contains three columns, shews the progress that was made in the work from day to day. The first column contains the date; the second, the spaces measured each day, reckoned by hundreds of yards, and denoted in the general plan by strong dots; and the third shews the temperature of the chain deduced from the mean of fifteen thermometers, or five for each chain.

Table of the Measurement of the Base of Verification on Romney

Marsh, executed in the Autumn of 1787..

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