gible. In addition to the sketch of the ground, a representation of the geological features of the country can be given, without at all. interfering with or confusing the sketch, by tracing on the back of 325, the paper the divisions of the geological features, the different porNations of which are afterwards coloured according to the conven tional system of distinguishing the several various formations on geological maps. On holding the sketch against the light these divisions appear clearly visible, though in any other position of the paper they are not in the least perceptible. Geological sections should also be shown on the margin of the sketch, having reference to lines drawn across it*. The inclination of such slopes as are of peculiar moment are Gmeasured with a "Clinometer," and the angles written either on the slopes themselves or as references. This little instrument can be made by cutting a small quadrant out of pasteboard and roughly graduating the arc. A small shot, suspended by a piece of silk, forms the plummet; and independently feet being made merely of pasteboard, fits into the pocket of the sketching portfolio. The slopes most necessary to note on a military sketch are those which relate to the facilities of ascent for artillery, cavalry, and infantry. According to the "Aide Mémoire," a slope of about 60°, or of 4 to 7, is inaccessible for infantry. 45°, or of 1 to 1, difficult. If 30°, about 7 to 4, inaccessible for cavalry. 15°, 4 to 1, inaccessible for wheel carriages. 5o, 12 to 1, easy for carriages. The leading features of ground are the summit ridges of hills (sometimes termed the water-shed lines), and the lowest parts of the valleys, down which the rain finds its way to the nearest rivers * The geological part of the Ordnance Survey is now quite distinct from the geodesical. or pools, called water-course lines. These two directing lines, if traced with care, will alone give some idea of the surface of the country, and assist materially in sketching the hills, particularly if drawn on the horizontal system, as the contour lines always cut the ridges and all lines of greatest inclination at right angles. It is a very common error, in first beginning to sketch ground, to regard hills as isolated features, as they often appear to the eye. Observation, and a slight practical knowledge of geology, inevitably produce more enlarged ideas respecting their combinations; and analogy soon points out where to expect the existence of fords, springs, defiles, and other important features incidental to peculiar formations. Thus appearances that at one time presented nothing but confusion and irregularity, will, as the eye becomes more experienced, be recognised as the results of general and known laws of nature. The representation of the outline of the hills, and their relative command, is also materially assisted in a topographical plan, and more particularly in a military reconnaissance, by a few outline sketches taken from spots where the best general views can be obtained. A series of these topographical sketches running along the length of a range of hills, and a few taken perpendicular to this direction, supply in some degree the place of longitudinal and transverse sections; and give, in addition to the information communicated by a mere section, a general idea of the nature of the surrounding country. A good judgment of distances is indispensable in sketching ground, even in filling up the interior of a survey, and more particularly in a reconnaissance, where there has not been either time or means for accurate measurement and triangulation. Practising for a few days will enable an officer to estimate with tolerable accuracy the length and average quickness of his ordinary pace, as also that of his horse (as on a rapid reconnaissance he must necessarily be mounted); and the habit of guessing distances, which can afterwards be verified, will tend to correct his eye. A micrometical scale* in the eye-piece of his field telescope, with a corresponding table of distances, is also a very useful auxiliary; and the gradual * See description of Dr. Brewster's micrometical telescope, in Dr. Pearson's Practical Astronomy, vol. ii. blending of colours, the angles subtended at different distances by objects of known dimensions, such as the height of a door, or a man, and the well-known rate at which sound has been ascertained to travel*, will all materially assist him. According to the "Aide Mémoire," the windows of a large house can generally be counted at the distance of 3 miles; men and horses can just be perceived as points at about 2200 yards; a horse is clearly distinguishable at 1300 yards; the movements of a man at 850 yards; a man's head clearly visible at 400 yards; and partially so between that distance and 700 yards. These directions, however, cannot be considered as infallible, as the power of vision differs so materially; but nothing can be more easy than for an officer to make a scale of this kind for himself. Another easy mode of judging distances is by marking on a scale or pencil held at some fixed distance from the eye, the apparent diameter or height, at different measured distances, of any objects the dimensions of which may be considered nearly constant; the average height of a man, a house of one or two stories, the diameter of a windmill, &c., will furnish suitable standards; and a short piece of string, with a knot to hold between the teeth, will serve to keep the pencil always at the proper distance. Suppose these scales to have been carefully marked for four or five of these objects, at the distance of 150, 200, 300, &c., yards, they will evidently afford the means of obtaining an approximate distance; but even without this scale, if the pencil b be held up to the eye at any distance a, and the height or diameter of any object h of * About 1100 feet in one second. A light breeze will increase or diminish this quantity 15 or 20 feet in a second, according as its direction is to or from the observer. In a gale a considerable difference will arise from the effects of the wind. A common watch generally beats five times in one second. See "Philosophical Transactions," 1823. The number of pulsations of a man in health is about 75 per minute. Either of these expedients will serve as a sort of substitute for a seconds watch. The velocity of sound is affected by the state of the atmosphere, indicated by the thermometer, hygrometer, and barometer; according to Mr. Goldingham, of an inch rise in the barometer diminishes the velocity about 9 feet per second. Mr. Baily rates the velocity of sound, at 32° Fahr., at 1090 feet per second, and directs the addition of 1 foot for every degree of increase of temperature above the freezing point. F 1. mensions be observed, then the distance from this object In rearing the outline of a work which cannot be approached thosely, for the purpose of tracing parallels and deterthe positions of batteries, the best plan is to mark, if pusslut the intersections of the prolongations of the faces and dks with the line on which the distances are being paced or Lustre, instead of merely obtaining intersections of the salient 40tering angles with a sextant. Soon after sunrise, or a before sunset, are the best times for these observations, as bgns and shades are then most strongly marked; in the middle of the day it is often impossible to distinguish anything of the outline of a work of low profile, even at the distance of 200 or 300 vards. If the perpendicular distance from the angle, or any other point of the face of a work, is required to be ascertained in the field; and the line marked on the ground for the purpose of laying out a battery, it can be readily done by the following method :— Fig. 1. Fig. 2. B D E Suppose, in each of the figures above, A to be the point from the distance is required on a line perpendicular to AB; meaistance CD, in a direction nearly parallel to AB, and es at C and D, formed by the line CD, and each of the 3, B being some marked object, situated anywhere on the line of the work, probably a salient or re-entering angle. From these data ascertain the values of A B, and the angle ABD, either by calculation or by any of the practical methods already described; BE is then the secant of the angle ABD to radius A B, and the difference DE between this quantity (to be found by means of a table of secants), and the calculated distance B D being laid off either on the line DB from D towards B (as in fig. 1), or on the prolongation of this line (as in fig. 2), the distance AE becomes the tangent of the same angle also to the radius A B; and the distance required for the battery can therefore be laid off on the ground by increasing or diminishing the length of this line AE. The direction of the capital of a work, and the distance from its salient, can be thus determined in the field. PRT equal to that observed at O; T being in the prolongation of SP. The triangles OSP and RTP are therefore similar, and the angle T being bisected by the line TV, it results that RP: PV :: PO: PX; which distance, laid down on the line PO, gives the point X required in the prolongation of the capital. The sides of the small triangle TPR and TV being all capable of measurements, OS, SP, and SX can, if required, be all found by a similar simple proportion*. It is, however, generally practicable to obtain a plan of any attacked work and of its environs, more or less correct; and on this * With a pocket or prismatic compass this operation may be more easily performed; by taking up a position on the prolongation of each face, and observing their inclination to the magnetic meridian, that of the line bisecting the salient, or the capital of the work, is at once known; for the mean between the two readings will be the bearing of the salient when the observer is upon the capital; and by measuring a base in a convenient situation, the distance may be readily found. F 2 |