In a book published by Messrs. Stanford, of Charing Cross, entitled "Ordnance Maps: Methods and Processes Adopted for their Production," the Ordnance datum employed for the operation of levelling is thus described:"The datum plane for Great Britain was determined in March, 1844, and is the height of the mean tide at Liverpool, according to the observations made at that time; this height has however since been proved to be '068 of a foot above the local mean level by calculations based on the mean of the recorded observations of four successive years, made with the self-registering tidal guage at St. George's Pier, Liverpool. "The datum plane for Ireland passes through a point, fixed on the 8th April, 1837, on Poolbeg Lighthouse, Dublin Bay, at low water mark of spring tides. "The adoption of mean water as a datum level for England in preference to low-water mark resulted from a series of tidal observations instituted round the coast of Ireland, in 1842, by the late Major-General Colby, which clearly showed that mean water is more nearly on a uniform level than low-water spring tides, and is consequently better adapted for a plane of reference for the altitudes of a general survey. As from the irregular conformation of the coast line, the local mean level of the sea varies considerably, the general mean for England was obtained by tidal observations made at thirty-two different stations, and for Scotland by observations at eighteen stations, in a similar manner to those previously made at Liverpool. connecting the mean found at each station with the levelling it was found that the relation of the local to the general mean varies in England from minus 1283 feet to plus 1.850 feet, and that the general mean level of the sea is 0.650 of a foot above the assumed mean level at Liverpool, which is the Ordnance datum." By Another bench mark employed, which is recognised in the Thames Valley, is known as Trinity high water. This is the level of the lower edge of a stone fixed in the face of the river wall upon the east side of the Hermitage entrance to the London Docks. No permanent mark is at present fixed to denote Ordnance datum, but Trinity high water (T.H.W.) line is taken as 12:48 feet above Ordnance datum. Colonel Sir Henry James, while director of the Ordnance Survey in 1861, published a book, entitled, "Abstract of Levelling in England and Wales," in which he states, "the datum level for Great Britain is the level of mean tide at Liverpool, as determined by our own observations, and it is eight-tenths of an inch above the mean tidal level obtained from the records of the self-registering tide gauge on St. George's Pier, Liverpool." Ordnance datum is thus seen to be an arbitrary level, obtained as above explained, the general mean tidal level of the sea round the coast of England being somewhat higher than the mean sea level at Liverpool assumed by the Ordnance Department. While at Liverpool the mean tidal level of the sea is 650 feet above Ordnance datum, at Dover we find it to be 975 above Ordnance datum. The range of the tides is given in a book annually published by J. D. Potter, of the Poultry, London, entitled, "Tide Tables for the British and Irish Ports," and the information therein contained is prospective for the year recorded. Ordinary spring tides and average spring tides are convertible terms. A difference in range of 3 feet found at one place over another shows that high-water ordinary spring tides are I foot 6 inches higher, and low-water ordinary spring tides I foot 6 inches lower at one place than the other. The tide charts of the English and Bristol Channels and of the entrance of the Thames, by Lieut. A. H. Percy, also published by Mr. Potter, are useful for reference, to show the exact courses of the currents in these channels. CHAPTER XIII. ENLARGING AND REDUCING PLANS. THE enlargement of plans by the aid of either the pentagraph or the eidograph can never be recommended where great accuracy is of importance. Where possible, the best way is to replot the whole survey from the field-book to the enlarged scale required. But it may be well, nevertheless, to describe these instruments, as they are of use under some circumstances. THE PENTAGRAPH The Pentagraph is usually made of brass, and consists of four flat bars, so fixed as always to form a parallelogram in all positions of the instrument. The instrument works as a jointed rhombus, in which two longer bars shown in the figure are united by a double pivot, which is fixed to the end of one of the bars, and works in two holes placed at the end of the other, forming a knuckle-joint connection. The pencil point is attached to one of these bars, and the tracer point to the other bar. The two shorter bars are fixed by pivots to the longer bars, and are also joined at their opposite ends in a similar manner to the joint uniting the longer bars. Several ivory castors support the machine parallel to the paper and allow it to move freely in all directions. The position of either the pencil or the tracingpoint is a fixture, while the position of the fulcrum attached to one of the shorter bars, and the position of either the tracing or the pencil point which is attached to the adjacent longer bar, can be shifted to suit the proportion required for reduction. This adjustment is effected by means of sliding boxes, which can be fixed at any part of the adjacent long and short bars by turning milled-headed clamp screws. The fulcrum contains a lead weight, to which is fixed a bright iron or steel pin, over which the whole instrument travels when in use, being balanced by the six wheels, which THE EIDOGRAPH rest upon the drawing paper or table upon which the instrument is working. It is important that the drawing paper should lie perfectly flat upon a level table, otherwise the wheels upon which the instrument is mounted will sustain frequent jerks, which will lead to inaccuracy in draughtsmanship. The principle of its construction is seen by experiment to be due to the fact that two points moving in a plane in any direction, but always remaining in the same right line with a fixed point and preserving the same proportional distance from it, will describe similar lines and figures. In the Eidograph, invented by Professor Wallace, of Edinburgh, in 1821, consisting mainly of three stout brass bars, these supporting wheels are dispensed with, and the G ELEVATION OF ARM MARKED B CARRYING THE PENCIL FIXED STAND ADJ INSTRUMENT ZERO FOINT NOTE TO SET THE INSTRUMENT, TO REDUCE OR ENLARGE TO ANY THE ONLY ADJUSTMENT WHICH MAY BE PUT OUT, IS THE PARALLELISA PASSING UNDER THE WHEELS THIS IS TO BE CORRECTED AS FOLLOW VERNIERS TO ZERO, AND WITH THE ARMS AT RICHT ANCLES TO THE CE MARK WITH THE TRACER AND PENCIL POINTS, THEN WHEEL THE INSTRUM AND PLACING THE TRACER INTO THE MARK MADE BY THE PENCIL, THE FALL INTO THAT MADE BY THE TRACER; IF IT DOES NOT, HALF THE ER CORRECTED BY LETTING OUT AND TAKING UP THE SPRING PASSING UND BY MEANS OF THE SCREWS IN THE MIDDLE OF THE WIRES. |