Description of the Apparatus made use of in the Measurement of the Base of Verification in Romney Marsh, with the hundred-feet steel chain, in the autumn of 1787, with the result of that operation. Reference to be bad to Plates VI. and VII.

Preamble.

19. In the account of the measurement of the base on Hounslow Heath in 1784, we had occasion to show, how very accurately distances might be determined by the steel chain, when applied in the ordinary way on the natural surface of the ground, if that surface happened to be tolerably smooth, which was the case in the instance alluded to. By the comparison of the measurement of a length of one thousand feet with the glass rods, and with the chain when used with an apparatus adapted to the purpose, it further appeared, that the difference between the results was so very small as scarcely to be discernible, since it would not have exceeded half an inch on the whole length of that base of 27404 feet.

Having always considered the experiment on Hounslow Heath, just now mentioned, as positive proof of the excellency of the chain, it had been resolved on to apply it to the mensuration of the base of verification in Romney Marsh, even if no other reasons had existed to make that choice eligible. But besides the danger of having the glass rods broken, in transporting to so great a distance from London, and, on such an event happening, the impossibility of getting them replaced with others, at the advanced season of the year in which we were unfortunately thrown with the operation, it was obvious, that in a plain of the breadth of six miles, so much intersected with ditches full of water as Romney Marsh in reality is, the laying of bridges for the tripod stands,

which must have been used with the glass rods, would alone have been a very troublesome and tedious operation.

Beech Posts.

20. In the first place, about thirty posts made of beech wood, three inches in diameter, and of different lengths, from two feet three inches to three feet six, and a few of them still longer, were provided. They were shod with iron, and each of them carried on its top a cast-iron ferrule, with two dovetails projecting from it; care being taken in driving them into the ground, that the dovetails should stand in or nearly in the direction of the base, as represented by the plan and section of a single post in the middle part of Plate VI. The arrangement of twenty-four of these posts may be seen at the top of the said plate, for the measurement of a portion of the base equal to one hundred yards, or the length of three chains. Sixteen of the posts, reckoning from that which stands in the centre of the first group, to that which stands in the centre of the second, and so on from right to left, were placed at the distance of twenty feet from each other. The first is supposed to coincide with the mouth of the pipe sunk into the earth, at the eastern extremity of the base, at a place called High Nook near Dymchurch; and every fifth post from that towards the left, marks the end of a chain. The other eight posts in the arrangement, that is to say, the right and left posts of each of the four groups, are supposed to stand twelve or fifteen inches from those in the centre. By referring to the elevation near the top, and the plans and section in the middle part of Plate VI. it will be perceived, that these posts, together with certain other iron parts of the apparatus fixed to them, hereafter to be described, support the ends of the coffering for each chain, free and independently of the central posts, to which last the brass scales alone are attached.

Deal Coffers.

21. Fifteen deal coffers, numbered from one to fifteen, were necessary for the length of three chains, being five to each. Six of them, that is to say, the first and fifth, the sixth and tenth, the eleventh and fifteenth, being the first and last of each chain, were only nineteen feet four or five inches in length. The other nine, being the three in the middle of each chain, were of the complete length of twenty feet. These coffers perfectly resembled in shape, and nearly in dimensions, the cases of the glass rods, being ten inches broad in the middle, and uniformly of that depth throughout their whole length. But from the middle they became gradually narrower, in a curvilinear manner, towards each end, where they were only two inches wide. The two cheeks or sides were about half an inch thick, and the bottom, which entered into a shallow groove in the middle of the cheeks, was an inch in thickness. Thus the cheeks being thin, bent and applied easily to the bottom, to which they were firmly nailed, and the whole was fortified by small blocks of wood fastened at intervals in the inside, sometimes above and sometimes below the bottom. From the elevation it will be perceived, that nine or ten inches of the under extremities of the cheeks were cut off, so as to permit the bottom itself to rest on the irons. This construction of the coffers was found to answer very well, that is to say, they were, considering their length, not so heavy as to be unmanageable, at the same time that by their general figure, and particularly the depth of the cheeks, they were entirely prevented from warping.

In addition to the fifteen coffers, just now described, a sixteenth, not represented in the plate, was afterwards prepared at Hythe, by Lieut. Fiddes, to be used occasionally, when the end of one chain, and commencement of another, coincided with a deep ditch or one of the sewers full of water, and where of course it would have been

extremely difficult, if not impossible, to have fixed steadily the group of three posts in the usual manner. In this coffer there was a double or false bottom, with grooves adapted for the purpose; and the brass scale, pulley, &c. were removed from the irons, and placed on this bottom.

Apparatus of cast iron, &c. for the Ends of the Chain.

22. By referring to the plate, where the several parts of the apparatus for the extremities of the chain are represented in plan and section, by a scale equal to one-fourth of their real dimensions, it will appear that the cast-iron pieces were of two different forms, one long, and the other short; but both applied in the same manner, on the ferrules binding the tops of the posts, as has been already mentioned. Of the long kind there were in all fifteen or sixteen, that is to say, one for each post in a length of three chains. Each iron had two clamps on its under side, which being slackened, it was placed on its ferrule at right angles to the line of measurement; and being turned round 90°, the dovetails of the ferrule, standing originally in the direction of the base, came within the clamps, which were then tightened by four screws, turned with square keys adapted to the purpose.

It is sufficiently obvious, that so many irons, with such a number of screws to each, could not fail of rendering this operation tedious! The business would have been greatly expedited if there had been only two such screws, one on each end, in a middle situation; and, instead of the four screws, there should have been four steady pins, entering easily into holes prepared for them in the under side. A short groove, of two or three inches in length, in each extremity of the bottom, would, on this supposition, have been necessary to suffer the square heads of the screws to pass; and it will be readily conceived, that the thickness of the

bottom would have effectually secured the chain from touching them, prevented the mutilation of its handles, and saved much loss of time. Indeed the same purpose might have been effected, but not so advantageously, by laying the original four screws lower in the iron, which its thickness easily admitted of. Finally, in order to avoid such like inconveniences in future, there is still one imperfection more, which it is incumbent on me to remark, namely, that cast-iron ferrules will not answer; for the force that was found to be necessary to drive the posts into the ground, burst almost the whole of them, so that before the operation was completed, they were obliged to be replaced with others made of hammered iron, forged for the purpose.

Of the short irons, only three were necessary, one for each end of the chain, and a spare one in case of accident. They were placed, turned, and clamped on the ferrules, in all respects similarly with those of the long kind. By inspection of the plate it will be seen, that each of them carried on its surface a brass scale of six inches in length, divided into inches and quarters, and moveable in a slide, either backwards or forwards, by a fingerscrew adapted to the right-hand end.

The right-hand post of each group is called the drawing-post, because the iron fixed on its top carries a small apparatus of brass, which being connected with the flat iron rod and hooks formerly used at Hounslow Heath, for a like purpose, lays hold of the rear handle of the chain, and draws it back until zero coincides with the point of commencement. The left-hand post in each group is called the weight-post, because it carried a brass pulley, over which a weight of 28 lbs. was hung by a small rope attached to the hooks that laid hold of the front handle of the chain. This weight acting against the force of the screw at the other end, the chain was thereby kept perfectly straight in the coffers, and con. stantly in the same degree of tension, until some certain division