"The slopes most necessary to note on a military sketch, are those which relate to the facilities of ascent for cavalry and infantry. According to the 'Aide Mémoire,' a slope of about 60°, or of 4 to 7 is inaccessible for infantry. If 30°, about 7 to 4, inaccessible for cavalry. 15°, 5°, 4 to 1, inaccessible for wheel 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 and 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, on first beginning to sketch ground, to regard hills as isolated features, as they often appear to the eye; observation, and a knowledge of the outlines 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 formation; and 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 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, when there has not been either time or means for accurate measurement or 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 micrometrical scale * in the eyepiece of his field telescope, with a corresponding table of distances, is also a very useful auxiliary; and the gradual 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 also materially assist him. * See description of Dr. Brewster's micrometrical telescope, in vol. ii. of "Dr. Pearson's Practical Astronomy." † "About 1140 feet in one second. A light breeze will increase or diminish this quantity fifteen or twenty 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 seventy-five per minute: either of these expedients will serve as a sort of substitute for a second's watch. The velocity of sound is affected by the state of the atmosphere indicated by the thermometer, hygrometer, and barometer; according to Mr. Goldingham, 1-10th of an inch rise in the barometer diminishes the velocity about nine feet per second. G According to the 'Aide Mémoire,' the windows of a large house can generally be counted at the distance of three miles; men and horses can just be perceived, as points, at about 2200 yards; a horse is clearly distinguished 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." |