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DESCRIPTION OF THE PEDIOMETER.*
For determining the areas of Plans without calculationwhereby a saving is effected of more than half the time now consumed in calculation, and the liability to error is very materially diminished.
Manufactured and sold by Troughton and Simms, 136, Fleetstreet.
The instrument consists of a square, and a graduated scale, constructed for three chains to the inch.
a_The milled head, by turning which, motion is given to the brass slider B, and the two pointers R and W.
I–The index to be placed in coincidence with the bom division upon the scale.
* The Lords of Her Majesty's Treasury have rewarded the ingenious inventor of this instrument Mr. F. Sheereck, Land Surveyor, Ewell, with a donation of One Hundred Pounds.
When the brass slider B is in contact with A, I coinciding with - division and R and W pointing to O upon their respective
R scales, the instrument is in adjustment.
When deranged, restore it, by opening R and W to the proper distance, and then moving A and I, the former into contact with B, and the latter into coincidence with
Required the content of the trapezium ECFD.
1st.—Place the edge A upon the point E, and open B to the point F.
2nd.-Press the square firmly down with the right hand, and with the left, place the scale against the edge of it, as shewn in the figure.
3rd.—Now press the scale firmly, and slide the square up, until the edge AB is upon the point C.
4th.—Press the square firmly, and slide the scale against its edge until - coincides with I.
Finally.-Press the scale and slide the square down until the edge AB is upon the point D, and taking out the numbers to which W and R point, subtract the latter from the former, and the contents in acres and decimal parts of an acre, will at once be given.
The red pointer directs to the numbers that are to be taken from the red scale, and the white one, to those upon the white scale.
When the pointers fall exactly upon the line engraved on the ivory edge of the scale, the folding leaf is to be doubled down to the left hand, but when the pointers fall between any two of the lines on the ivory edge, the folding leaf must then be doubled over to the right hand before the numbers are read off.
For instance, when the leaf is turned to the left and the red pointer falls between the two lines which refer to .009 and .013, turn the folding leaf to the right hand, and the pointer will read 0.10,
It will be found most convenient and most accurate in practice to take the shortest diagonal for the line EF.
The basis of an accurate survey, undertaken either for any extensive geodesical operation, such as the measurement of an arc of the meridian, or of a parallel ; or for the formation of a territorial map, showing the positions of towns, villages, &c., and the boundaries of estates and counties; or a topographical plan for military or statistical purposes, must necessarily be an extended system of Triangulation, the preliminary step in which is the careful measurement of a base line on some level plain : at each extremity of this base, angles are taken to several surrounding objects previously fixed upon as trigonometrical stations; and also, when practicable, the angles subtended at each of these points by the base itself. The distances of these stations, from the ends of the base line and from each other, are then calculated, and laid down on paper, forming so many fresh bases from whence other trigonometrical points are determined, until the entire tract of country to be surveyed is covered over with a net work of triangles of as large a size as is proportioned to the contemplated extent of the survey, and the quality and power of the instruments employed. Within this principal triangulation secondary triangles are formed, and laid down in like manner by calculation; and the interior detail is filled up between these points, either entirely by measurement with the chain and theodolite, or by partial measurement (principally of the roads],
and by sketching the remainder with the assistance of some portable instrument. The degree of accuracy required, will of course determine which of these methods is to be adopted: the former is practised on the Ordnance Survey of Ireland, now in progress under the direction of Colonel Colby, on a scale of six inches to one milė, but the latter was considered sufficiently minute for that of England, which was first plotted on the scale of two inches to one mile, and afterwards reduced to one inch to one mile.
In a flat uncleared country any attempt at a system of triangulation would be useless. In such cases the only mode of ensuring tolerable accuracy in surveys of great extent is that which has been generally adopted in the construction of geographical maps. The latitude and longitude of a number of the principal and most conspicuous stations are determined by astronomical observations, and the distances between them calculated to enable their positions to be laid down as correctly as they can be determined by this mode of fixing the relative place of each station. In surveying any extended line of coast, where the interior is not triangulated, no other method presents itself; and a knowledge of practical astronomy therefore becomes indispensable in this, as in all extensive geodesical operations. A topographical survey further requires that some of the party employed upon it should be well versed in the general outlines of geology; as a correct description of the soil and mineral resources of the different parts of every country forms one of its most important features. The heights of the principal hills, and of marked points along the ridges, plains, valleys, and watercourses above the level of the
sea, should also be determined, which on the Ordnance Survey of Ireland is done by levelling with the theodolite. In a survey of less pretensions to correctness in minute detail, the heights may be ascertained with tolerable accuracy by means of the mountain barometer.
A sketch of a certain tract of country, on a far larger scale than that of most general maps, is constantly required on service, for the purpose of showing the military features of the ground, the relative positions of towns and villages, and the direction and nature of the roads and rivers comprised within its limits. This species of sketch, termed a Military Reconnaissance," approaches in accuracy to a regular survey, in proportion to the time and labour that is bestowed
CARRYING ON A TRIGONOMETRICAL SURVEY.
upon it. Having thus adverted briefly to the progressive steps in the different species of surveying, they will each be treated of more in detail in their proper order.
The system of forming the “net work of triangles ” alluded to, of as large a size as is consistent with the circumstances under which the survey is undertaken, is to be considered as the working out of a general principle to be borne in mind in all topographical and geodesical operations, the spirit of which is always to work from whole to part, and never from part to whole.
By the former method errors are subdivided, and time and labour economised; by the latter, the errors inseparable from even the most careful observations are constantly accumulating, and the work drags on at a slower rate and an increasing expenditure.