APPENDIX B.

AN ACCOUNT OF A RUNNING SURVEY OF THE RIVER IRRAWADI,

By LIEUTENANT HEATHCOTE, H.M. Indian Navy.

A RUNNING survey of the Irrawadi river above the frontier of the British possessions in Pegu was executed on the occasion of the ascent of that river by the diplomatic mission despatched by Lord Dalhousie in 1855, to make return presents to the King of Ava, and to negociate a treaty of commerce. The mission started in two flat-bottomed river steamers, drawing between three and four feet water, each towing a flat or large barge of about equal size and draught with the steamer. On the British frontier where it abuts on the Irrawadi, two boundary pillars are erected; their relative position and distance from each other has been well ascertained, and the river within this boundary has been well surveyed. It was the duty of the surveyors of the mission to produce as accurate a survey of the river to the north of this boundary within the territories of the King of Burmah as the time occupied in the passage of the mission allowed them. The principle laid down for the execution of this duty was a combination of the three elements of time, speed, and transit bearings; the result being checked and confirmed by astronomical observations. The surveyors were well provided with instruments, and five chronomers were placed in a cabin of the largest flat, upon a stand purposely erected, to avoid vibration, or the effects of concussions, and to give every facility for winding and comparison. At each end of the roof of the flat, which was about 15 feet above the water, and 120 feet long, standards were set up to give a line of sight at right angles to the keel of the vessel. These were used to ascertain the rate of progress, by noting the inverval between the passing by the two sets of standards of any fixed object on the bank, when the vessel's course was perfectly straight. A good prismatic compass was set up on the roof of the flat; and, starting from a given point, and noting time and speed, the transit bearing of every point or object (especially those on the bank) with every other that was worth remark was accurately noted. Objects in the interior were observed in the same way, and these observations, both backwards and forwards, were made as numerous as possible, so as to act as a check one against another. Time of arrival, and speed, at every point or object before observed, was again noted, the course being principally along either one bank or the other, to avoid the extreme strength of the midcurrent. The eventual plotting of the chart was throughout kept constantly in view, and the connection between all the various objects carefully preserved. To avoid the errors, to which observations of the speed as tested by the standards were liable from any accidental deviation from the vessel's straight course, these observations were taken very frequently, and

may induce and facilitate the prosecution of much higher studies, such as Geology, Military History, Landscape Drawing, Geographical and other reading; while, if prosecuted with zeal and ability, it often enables officers to amass information of the most valuable character in foreign countries. Favoured with such opportunities, they may employ their leisure time most profitably; and, lastly, they may be induced to take a special interest in our national works of survey and topography, which are second to none. I point to these our Ordnance Surveys, our military reports respecting foreign possessions, and the present labours of the Topographical Department of the War Office, because, as in all matters requiring active exertion, industry, and excellence, in Art generally, it is both necessary and encouraging to the beginner to have some standard of excellence works in which he may at once feel a legitimate pride, and whose recognised value stimulates to continuous exertion. To become in a measure identified with these national works is an honour of itself, and at the present day the original sketches sent home by officers serving abroad are received at the Topographical Office by Sir Henry James, and published in the highest style of art.

The instruments in use in Military Sketching and Surveying are very well described in the books referred to (Jackson and Drayson); you cannot become familiar with them at a glance, nor by any cursory description which I could now give. Practice alone will render their use easy and satisfactory, but with that, you may in a short time acquire great confidence in handling them. The theodolite will best impress upon your mind and memory the system of angular measurements, and, being fixed on a tripod, it can be worked in a convenient manner. The smaller instruments for measuring horizontal angles, viz. sextant, prismatic compass, &c. &c., in more general use among military sketchers, are on rather a small scale for beginners. I find great value in teaching first principles with a theodolite.

Supposing now that we have learnt the uses and manipulation of the instruments already referred to; let us imagine we are about to make a survey of a small tract of ground, say about 6 square miles. We are about to take such measurements on the ground as shall enable us to mark down on paper the relative position, dimensions, and character of objects on the surface of that ground. We consider first what sized plan we are in want of. Shall it be a large plan or a small one? shall it embrace minute details or not? The more you wish to have recorded the bigger your plan must be. Your work will be on a large scale or a small, according to ultimate ends in view. This determines the Scale of the plan. Let us take a good medium scale, say six inches to a mile; that point settled, every line on paper six inches long will be held to represent one mile on the ground, and our design will now be to plot, or mark on paper all the objects found on the ground, so that their relative positions shall be preserved, To do this, we are not obliged to measure the exact distances between every two or three objects; we measure one good line somewhere on our ground (about the middle, if possible). We do this with a rope, tape, or chain, or with still better contrivances (Compensation bars, &c.), according to the nature of the work on hand.

From the two ends of the line A B, Plate I., we could determine

the exact position of the point C if we actually measured up to it by chain; but can we not adopt a simpler method? C is the apex of a triangle of which A B is the base (already ascertained); now find the angles made with A B by lines directed on C from A and B; you then have one side of a triangle (measured), and two (or all) of its angles; Trigonometry will do the rest. You can get the exact position of C without ever going near it, by the properties of triangles. Extend this plan of operations so as to arrive at the position of D, E, F, &c., and any number of points or places on the ground, and you have what we call a triangulation.

There are instruments called protractors, which enable you to protract or lay down angles on paper so accurately, that for Military Surveys (works of limited extent on small scales) it is not always necessary to calculate positions of C, D, &c., for they can be accurately determined by the intersection of the protracted bearings. Observe now the uses and value of your triangulation. It simplifies your work of linear measurement; it establishes from a central well-chosen line, which can be very accurately measured, the exact position of many points or places on the confines of your ground, or on its more remarkable features. It not only saves time and trouble, but it ensures accuracy, makes a skeleton network to brace together your survey, saves a multiplication of errors sure to result in measuring long lines, and furnishes also lines of direction across your work which greatly facilitate its execution.

In applying these principles practically, we first carefully walk over and examine well the ground, and choose some fitting locality for the measurement of a base-say, about 1,000 yards in length for a six-mile survey. It is usual to look for a very level place, because a line along it can be measured to a nicety. The extremities of this line must be clearly visible, each from the other, and they must afford extensive views of the surrounding country, in order that observations may be made of distant stations. Then, by the use of instruments, we ascertain the relative positions of these points by triangulation.

It is a point not enough dwelt upon by authors of works on Military Surveying, that from each end of the base there should be a very clear view the ends, in fact, should be well exposed, not hemmed in by heights, woods, or other large masses circumscribing the horizon. By simple intersections, work can be plotted much more accurately than is commonly supposed. It can be plotted on the scale or scales commonly used in military surveys quite accurately to the extent of the observations themselves, provided a certain number of checks are used; and as this method saves much time, it is very desirable to commend it to a certain degree, as much of the value and usefulness of a military sketch may depend upon the promptness with which it is handed in to a commanding officer. The extreme nicety of measurement of survey "bases" may well be studied by all interested in the subject, and some one wellapproved system must be practically carried out, whenever it is proposed to execute a Survey of considerable magnitude.

Having established a base, measured it from each end, and made observations of all the best stations you can fix upon in the surrounding country, you may proceed to cut up the work into secondary triangles,