to the magnetic meridian at all, as a line connecting the starting point, a, with any visible fixed object, may be assumed as a working meridian; and, if it be thought necessary the reading of the compass needle may be noted at a, when such fixed object is bisected; and upon the theodolite being set to the reading of this assumed meridian, at any subsequent station, the compass needle will also point to the same reading as it did at first, if the work is all correct, and no local attraction influences the compass.

While the instrument is at A, take angles to all the conspicuous objects, particularly to such as you may hereafter be able to close upon, which will (as in the above instance) verify the accuracy of the intervening observations; having done this, return to g and f, &c., and proceed with the survey in the same manner as before, setting the instrument up at each bend in the road, and taking off-sets to the right and left of the station lines; arriving at i, survey up to, and close upon, B; then return to i, and proceed from station to station till you arrive at m, where, if the whole work is accurate, the forward angle taken to b will be the same as was formerly taken from b to m, which will finish the operation.

SECTION VIII.

GENERAL OBSERVATIONS ON SURVEYING.

SURVEYING may be defined, the art of representing a country, or any portion of the earth's surface, upon paper, in such a manner that we may be able, by means of a scale, to measure the horizontal dimensions of its features; as territorial boundaries, lakes, rivers, forests, roads, &c.

The foundation of every survey is a base line; and upon the accuracy with which the length of the base is obtained, the correctness of the entire survey must depend.* The measurement of a base, then, is not only the primary, but also the most important work to be performed. Various methods have been pursued to effect this object.

Previous to the measurement of the Hounslow base, deal rods were considered very good for the purpose, and had been extensively used on the Continent; but experiments made on that occasion proved their unfitness, owing to the uncertainty of their expansion; perhaps, however, our more humid climate may have produced a greater effect

* The length of the base measured on Hounslow Heath, for the grand Trigonometrical Survey of England, in 1784, was, when reduced to the level of the sea, 27404-1037 feet. Three modes of measuring were tried, viz., with deal rods, with a steel chain of peculiar construction, and by means of glass tubes. The uncertain expansion and contraction of the deal rods, was found to produce a very fallacious result. The steel chain does not appear to have had a fair trial upon this occasion. The method proposed with glass tubes obtained the preference. Accordingly, the measurement took place with these, and was conducted in a scientific manner, with all the care due to so important an operation. In 1791, a careful measurement of the same base was made with the improved steel chain, which only differed from the original one performed with glass tubes, by about 2 inches. Subsequently, in 1794, a base of verification was measured on Salisbury Plain, which varied only about 3 inches from its computed length.

upon them, than the comparatively dry atmosphere of situations where they had before been tried; and it appears, from the account of the operations on Hounslow Heath, in 1784, that the season was a very wet one. On the whole, if I may presume to offer an opinion in the matter after their condemnation by men of so much science, I should still advise the use of deal rods under other circumstances, and where glass tubes and steel chains, such as were used on the occasion above mentioned, with all the vast preparation made for their application to the object in the most perfect manner, cannot be had; and, more than all, when the contemplated survey is not of the most important character.

In ordinary surveys, it is not necessary to enter into calculations, on account of the sphericity of the globe; nor indeed into many other niceties, such as would be imperative, were the object to measure an arc of the meridian, or perform any other grand geodesical operation. On common occasions it suffices to consider the earth as a plane or flat surface, and all the sides of the triangles as right lines, instead of curves.

The ground having been selected for measuring a base, and this operation performed with all the accuracy the means will admit of, the next step is to choose the most eligible points for carrying on the triangulation—a survey being conducted by means of a series of triangles, of which the base line forms one side of the first. With this view, conspicuous situations are fixed on, as the tops of hills, church towers, &c. These primary stations ought generally to be of a distance from each other, bearing some proportion to the length of the base and extent of the proposed survey. For instance, if the base be two miles, and extent of the survey 15 or 20 miles, the sides of the triangles may be from two to four miles; much, however, must always depend on the relative position of commanding points for

stations, and on the two first triangles of the survey. For example:-Suppose A B to represent a base line, and that

C and D are eligible stations, forming two triangles, A CB and AD B. Knowing the length A B, and the angles at A and B, the length of C D is found by an easy calculation in trigonometry; and that line becomes nearly as good a base, in point of measurement, as A B, while it possesses the advantage of being longer, and thus enabling us to increase the sides of our triangles.*

C

B

Wherever the instrument is set up, observations should be taken to all remarkable objects; these, being repeatedly intersected, furnish a check on the work as it proceeds, and their several positions are furthermore determined for future use.

When possible, all the angles of the principal triangles should be observed; then, as the sum of the three angles ought to be 180°, we are enabled to judge of the accuracy of the observations, and in some degree of the perfection of the instrument used.

The sides of all the principal triangles should be calculated, and laid down by means of beam compasses—as laying down by the sides is always more correct than by the angles. In triangles on a large scale, an error of a single minute, in protracting an angle, would sensibly affect the length of the sides.

As it is impossible to avoid some degree of error in taking angles, we should endeavour so to order our opera

* This method of obtaining a longer base, as it may be termed, becomes useful when a base is measured on low ground between hills, as must frequently be done; such situations being often level, and suitable for the purpose.

tions that the error may have the least possible influence on those sides, the exact measure of which is the object to be obtained.

When the base cannot be equal to the side or sides sought, it should be as long as possible; and the angles at the base should be nearly equal.

Sometimes it occurs that the three angles of a triangle cannot be observed; in that case, the angle obtained by intersection should be as near as possible a right one. Acute intersections are at all times to be avoided.

The fewer the principal stations, the less will be the labour of the survey; it will also be more accurate, and less liable to mistakes while in the field, or errors when plotting the work at home.

Military men generally fill in the principal triangles by means of the surveying compass, when the survey is not required to be minutely exact in all its details.

These general observations might be multiplied to an unlimited extent; and yet, after all, when a survey is to be undertaken, the surveyor must depend chiefly on his own judgment, to lay out the work to the greatest advantage, according to the nature of the country, and other circumstances that will affect his operations.