THE paper must be ruled with parallel lines at unequal distances, as before.

Previously to commencing the sketch, it is best to go over the ground and examine it, so as to fix a plan of it on the mind. Then select a few prominent points for stations, from which bearings are to be taken to all conspicuous objects contained within the limits assigned to the sketch; such as houses, remarkable trees, a windmill, church steeple, &c. Next, select the longest and most level space on which to measure a base, which may be either on the summit of the ground you are to sketch, or along the bottom, as most convenient.

Plate III. represents a hill, partly surrounded by a road, of which our object is to take a military sketch with some degree of accuracy. On inspection, it is found that the direction A B is most favourable for the base line. E, D, and C, are situations eligible for stations. Now, the compass being placed at A, the bearing of E is taken, namely, 61° E. D is found to bear 106° 30′ E. B, 172° 20′ E. C, 24° 30′ W. Protract these several bearings, by marking a point, A, on the paper, in such a situation as will admit of the whole contemplated sketch being contained on the sheet, and adjusting the ivory protractor to the lines, as on the

*The term ground is generally to be understood as applied in contradistinction to a flat or level; so, whenever it is met with in the course of this work, hills or elevation will be meant.

former occasion, which will then lie in the position that is shown in the figure. Measure the base A B. The mean of three times pacing it over gives, suppose, 1130 paces, of 30 inches each. Lay your scale along the line which indicates the bearing of B, and mark off the number of paces, which fixes the point B.

Again, plant the compass at B, and take the bearing of C, 124° W.; of A, 172° 20′ W.; of E, 11° E.; of D, 22° 30' E. It will be remarked that the bearing of B from A, and that of A from B, give the same number of degrees and minutes: they form, in fact, alternate angles with their respective meridians, and present, when laid down, the same straight line; furnishing a proof that the bearing taken at A was correct.

The several bearings taken from B are now to be protracted, and the intersections with the lines drawn from A, fix the points C, E, and D.

The observer should now look around, and notice what objects are visible that it may be advantageous to fix, with a view to assist him in sketching, and filling in his work. For this purpose a bearing must be taken to the farmhouse; also to the point G, where a road turns off. The farm bears 89° E., and G, 50° 30′ W. These two bearings are then to be protracted. Proceed with the compass to C, and take the bearing of G from thence, 6° W., which, on being laid down, cuts the bearing taken from B, and determines G. While at C, it is perceived that the point F, where a road branches off, must be fixed: a bearing is therefore taken to F, namely 175° 30′ W. The compass is then carried to D, and the position of the farm-house is decided by a bearing that cuts the one taken from B.

It is a maxim in surveying, that when fixing any points of importance, both acute and obtuse intersections are to be avoided as much as possible. The nearer an intersection

approaches to a right angle, the better: hence, in selecting primary stations, we should endeavour to have them so placed, that bearings which determine them shall intersect or cut each other as near at a right angle as circumstances will permit.

This observation applies particularly to the kind of rough surveying now under consideration; for it is manifest that some degree of error must attend every operation performed; for instance, the base line is measured by pacing—our compass only gives a bearing to within three or four minutes of the truth-the protractor is even more inaccurate, and, with every care, some error will attend its adjustment to the parallel lines. Now, if to all these unavoidable sources of error, we add such as will attend a very acute intersection, primary stations so determined, from which other points in the sketch are afterwards to be derived, must lead to great incorrectness.*

Referring to the sketch before us, E, standing on an elevated knoll, is higher than any other of the stations; and supposing the sketch extended beyond the limits here assigned to it, the point E, from its superior elevation, which enables us to look well around, ought to be carefully fixed. In this view, the lines A E and BE intersect too acutely for the degree of accuracy that is desirable, and ought therefore not to be solely trusted to; especially as station D furnishes an opportunity to correct the position of E. When at D, therefore, the bearing of E is to be taken for this object.

Lastly, the bearing of F is observed from E, and the line C F is intersected, thus fixing F where the roads meet.

* The student is not to be alarmed at this array of causes productive of error. In a sketch comprehending an area of a square mile, the most distant points will not be generally more than 30 or 40 paces out-a matter of little moment in a military sketch. But it was necessary to warn him on the subject.

The contours of the hill should be sketched in during the foregoing operations. Thus, after protracting the bearings taken at A, the declivities near that station are sketched in, having reference to the lines laid down. In the direction of C, for instance, we find that at 140 paces a descent begins, and at 145 paces further we regain the summit-still walking direct upon C. When C is fixed, a curve is swept round to meet what was sketched from A; and thus the business proceeds. But I shall not dwell longer on the sketching part in this place-proposing, in a subsequent section, to try and render the process intelligible to the student.

With regard to the road in our sketch, we find three points on it laid down, namely, F, G, and the farm-house: the intermediate portions of it are either sketched in by the eye alone, or bearings may be taken along it, in the manner we have already seen (Section II.), to close from point to point-as from F, closing on G, and from G, on the farm.

If it were wished to carry the sketch over a greater extent of country, bearings to distant objects could be taken from points already fixed, as E, F, G, &c., and their several situations determined; and thus a net-work of triangles might be formed, similar to those of a trigonometrical survey. Indeed, our sketch is a minor description of trigonometrical survey. There is the base line A B. The primary stations, A, E, D, B, and C, with triangles formed by their bearings, laid down from each end of the base. Nothing can be more simple and easy than such an operation; and, notwithstanding the numerous sources of error, above stated, a few square miles of country may be laid down in this manner with sufficient accuracy for every military purpose.

A scale should accompany each sketch, and a meridian line must be laid down.