Town Surveys performed usually by "Traversing."

It is evident therefore, that surveys which are to be plotted on such large scales must be made with the utmost care. They are usually performed by a process similar to that described under the head "Traversing," the theodolite being used to take the bearings of all the streets.

Town Surveys performed more accurately and expeditiously by the aid of Triangulation and Angular Intersections.

The work will be much expedited, and rendered more accurate, by taking angles from spires or commanding heights to remarkable points presented by the buildings public and private, such as observatories, remarkable roofs, projections, chimneys, &c., noting each object in the fieldbook at the time of observation by slight sketches of each object observed, sufficiently exact to serve as a means whereby it may easily be recognized. Such intersections taken from three points, themselves fixed in position, will give the site of the principal buildings; and when these are laid down, "traverses" run in the intermediate streets and roads will give the details with accuracy, the angular intersections serving as checks on the directions of the traverse lines.

Offsets and Measurement of Detail taken with the
Measuring Tape or Staff.

In town surveys the measuring tape is much used for the offsets, and especially for the measurements of the rears of houses and yards and small enclosures, through which the chain cannot be taken, to follow a station line.

Principal Lines of Construction only to be shewn on
the Plan.

In rural districts the surveyor is recommended to retain on the plan (showing them in red ink) all the lines of construction. For the plans of towns this practice would lead to confusion owing to the multiplicity of lines required. Those only, therefore, which mark the directions of leading streets, and form (as it were) great bases on which the remainder of the detail depend, should be marked.

"SKETCHING IN" DETAILS FOR MILITARY AND OTHER SURVEYS.

The completion of surveys, in which many principal points are determined, and of which a portion of the outline has been measured and plotted, and which are not designed to be accurate in the details, such as travelling maps, &c., may be expedited by taking the plan itself to the ground, and measuring with the box-sextant the angles between fixed points, or taking their bearings with the meridian by the prismatic compass; the directions of the lines and the measured distances being at once protracted on the plan, no field-book is kept. A protractor scale is required to set off the angles or bearings; one of its edges being divided as a plotting scale suited to the scale on which the plan of the survey is to be drawn. When plans are thus required to be taken to the field, they should be traced on bank-post paper, which admits of being folded without injury, so as to expose any portion of the plan, and only that portion which may be required at one time. In order to enable the surveyor to draw or plot more neatly, the paper may be folded over a rectangular piece

of board, fitting on a field sketch-book or portfolio, and retained in place by leather bands at each corner.

It is not unfrequently of advantage (to the military surveyor especially) to be able to construct a plan, or to fill in the interior detail between fixed stations, with tolerable accuracy, without instruments. By pacing, distances may be measured; bearings or directions taken with the aid of a straight walking-stick; and the distances plotted at once on the field sketch-book by the aid of a plotting scale-the intention of such reconnoissances, and their chief recommendation, being, that the plan may be constructed at the same time that the surveyor walks over the ground. The greatest difficulty consists in setting out correctly on the field-sketch the first few objects; their relative distances should therefore be paced, and their bearings taken with care. They may serve, when determined, in giving the position of other points or objects by the intersections of two or more lines passing through these objects fixed in position. The expeditious method of measuring distances approximatively by pacing, is not so liable to error as might be supposed, if due precaution has been taken to ascertain the length or value in feet or yards of a given number of steps made at the common walking pace. An attempt to make each step equal to one yard would produce greater inaccuracies, because of the difficulty of preserving for any length of time the steps of an unusual length. It is better to ascertain their length by actual trial, by pacing several times a known measured distance at the usual rate and pace. A very general measure of the length of steps is 24 steps to a chain, or to 22 yards.*

* It may not be out of place here to describe a method of measuring inaccessible distances, especially the width across a ford or a river, which experience, unassisted certainly by geometrical knowledge, has

Independently of pacing, practice in sketching outline plans trains the eye to judge short distances with tolerable accuracy. A knowledge of the laws of perspective, both linear and aërial, is also of some assistance; for as perspective teaches to transform actual into apparent forms, so it may teach to deduce actual from apparent forms, and thence obtain their true dimensions.

taught the peasantry of some countries to devise*. They place the top of a stick resting on the foot or knee, and kept as steady as possible in a line with the eye and the remote inaccessible object whose distance is required: they then turn the head and body round, and watching the point where the ray passing from the eye to the top of the stick intersects the ground on an accessible spot on the same level, they pace to the point of intersection thus found, and thereby obtain an approximation to the distance required.

* MALORTIE'S Topography.

CHAPTER VII.

ON SURVEYING AS APPLICABLE TO THE COLONIES.

COLONIAL surveying is distinguished from the usual land surveying, previously described, by a marked difference in its objects. In cultivated countries in which every portion of the land is claimed by a proprietor and an occupier, and the surface of which is divided into estates with known boundaries, or separated into legal and ecclesiastical divisions, the business of the surveyor consists in making, on a plan, a faithful representation of the existing demarcations and artificial objects, as well as of the natural features, and in collecting and arranging all data which may contribute to convey a knowledge of the physical aspect of the country. In new colonies, on the contrary, the first purpose of the surveyor, instead of being directed to the measurement of existing lines or boundaries, consists in actually setting out on the ground the limits of stated quantities of land or "sections," previously to their being conveyed to the purchasers.

Bearing in mind this difference, we proceed to describe a suitable mode of conducting such a survey.

When treating of trigonometrical surveying, it was explained that by it only could perfect accuracy be attained in the survey of an extensive district; but, at the same time, the description of the mode of operation made it manifest that it necessarily involves, in its prosecution, both considerable expenditure of money, and great consumption of time. In a new country, probably covered with timber or dense and tangled underwood, intersected by impassable rivers, and inaccessible marshes, and presenting other serious physical obstacles, the consumption