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intelligible, boundaries are marked by clearly defined strokes and dots, and territorial demarcations by characteristic styles of writing for the names, which render them plainly distinguishable. A table of comparative characteristics and symbols in use on the six inch and scales is given (pages 135-137), but of course upon a plan the thicknesses are drawn less than that shown upon this illustration.
Double lines of railway are shown on the six inch maps by parallel lines with cross strokes. Single lines of railway on the six inch scale are represented by a single line with strokes crossing it; on the scale both lines are shown. All first class public roads are shown by the east and south sides from the light being shaded. Occupation or private roads are not shaded. Rivers are marked by the side next the light being shaded, upon the west and north sides. The light is conventionally supposed to enter a plan from the north-west corner. Canals are distinguished from roads by the parallelism of the sides, by the locks and bridges, and by having the side next the light shaded like rivers.
In dealing with the enclosures, braces indicate that the spaces so braced same reference number. Arrows the direction of the flow of water. in the boundaries, the symbol point at which the change takes place, as the boundary may change in character.
on the plans are included under the along streams show When a change occurs is used to show the
Sites of antiquities are shown by the symbol
and to show the periods, the names of such are written thus:ROMAN Prehistoric or Saxon – Norman or Subsequent.
Altitudes are given in feet above the Approximate Mean Water at Liverpool. Those indicated thus- B. M. 5'47 refer to marks made on buildings, walls, &c., and are called Bench Marks. The use of the very appropriate barbed head or broad arrow as the Ordnance mark for levels upon walls and buildings is thought by some to have originated in its having been the crest of a former Master General of the Ordnance (Notes and Queries, Dec., 1872)..
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. By 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 1850 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."
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.
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 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