a much larger scale than the latter, otherwise the undulations on the surface of the ground would, in many cases, be hardly perceptible. Draw the horizontal line A G, setting off the distances A B. A C, &c., as in the column of distances, that is, AB = 4.60 chains, A C = 7.80, &c.; then draw A a 100 feet, perpendicular to A G and parallel to A a draw Bb, Cc, &c., setting off their heights 97-85, 91.10, &c., respectively from the column of reduced levels, and through the points a, b, c, &c., draw the required sectional line ag. NOTE.-The above operations, though extremely simple, require great care, otherwise, in extensive works of this kind, errors creep in imperceptibly, to check which the agreement of the differences in the level-book is essential. RUNNING A CHECK LEVEL. (15.) We shall now give an example of a simple kind to lead the way to more complicated operations. When a section of a line of country has been completed, it is in most cases necessary to check its accuracy by repetition; but in doing this it is seldom requisite to level over precisely the same line of ground, unless there is cause to suspect its general correctness, but to follow the most convenient and nearest route, and at intervals to level to some known points on the exact line of section, which will give their differences of level: the points thus selected are generally what are called bench marks, being notches cut on gate posts, stumps of trees, mile or boundary stones, or any similarly immoveable objects, contiguous to the line of section, and at frequent intervals. These bench marks are made by the person who first takes the section, and are sometimes previously determined upon. When the section is complete, their relative heights with respect to the base or datum line of the section becomes known; hence they may be considered as so many fixed points on the line, easily recognisable, from whence any portion of the work may be levelled over and over again. From what has been said it is clear that in taking running or check levels, the use of the chain and compass attached to the level is not wanted, the distances and bearings having all been previously taken. An example of this kind of operation is represented in Plate I., where both the ground plan and section are shewn. The strong black line on the plan is that of the section to be checked, and extends from a bench mark at the town A, in a winding direction, to another bench mark in a town B; this originally formed a portion of a more extensive survey. The route taken in proving the work is shewn on the plan by a dotted line, and was confined to the public roads, as being the most convenient route, especially as it crossed the line several times, by which a number of intermediate points could be checked. Previous to giving the particulars of this example, the method of performing the operation of levelling shall be explained more in detail than has yet been done. The staff-holder must first place his staff perpendicularly on the bench mark from whence the levels are to commence; the surveyor must next set up his spirit level on the most convenient ground that presents itself, and so that he may have an uninterrupted view of the line he intends to level; the station chosen should not be more than four or five chains from the staff-holder, where, having fixed the legs of the level firmly in the ground, the surveyor must adjust his level for observation in the following order:-"first he must draw out the eye-piece of the telescope till he can see the cross wires perfectly well defined; then, directing it to the staff, he must turn the milledheaded screw, on the side of the telescope, till he can likewise distinguish, with the utmost possible clearness, the smallest gradations on the staff: that these two adjustments be very carefully and completely performed, is of more consequence than is generally supposed, for on them depends the existence or non-existence of parallax." "The adjustment of the eye-piece to obtain distinct vision, when once properly made, is not likely to require alteration for the whole day, unless it be accidentally deranged; but that of obtaining distinct vision of the distant staff (together with the one we shall next describe) must be performed at every station as it varies with the distance of the staff. "Having made the above adjustments perfect, bring the spirit bubble to the centre of its run, which position it must retain in every direction of the telescope; or, in other words, the bubble must indicate a true level during the time the telescope is turned completely round; this is accomplished by bringing the bubble successively over each pair of parallel-plate screws, and giving them motion by screwing up one while unscrewing the other to a like extent; but if the telescope is supplied with a cross level, as in that of Gravatt's, the two bubbles being at right angles to each other, they will at once shew which pair of screws require turning, in order to indicate the true level. "The level being now adjusted for observation, it must be directed to the back staff, and with all possible exactness the foot and decimal fraction of a foot must be noted, with which the central part of the horizontal wire appears to be coincident, which enter in the proper column of the level book. This column should be headed Back Sights' as in the preceding example (Art. 14). As soon as it is registered, see that the bubble has not removed from its central position, and then repeat the observation to ensure that no mistake has been made: this should be invariably done to guard against error.” The telescope of the level must now be turned round to the fore staff, the object glass and bubble being again adjusted, if required, in the manner already described, and the reading of the staff observed and entered in the level book in the column headed "Fore Sights," verifying the observation as before by seeing that the bubble is still in its place, &c. It may be here proper to remark, that the surveyor's merely walking round the instrument, or accidentally striking one of its legs with his foot, will derange the level indicated by the bubble, especially where the ground is newly tilled or soft and spongy: therefore, the legs of the instrument should always be fixed firmly in the ground. To do away with this inconvenience, as far as possible, Gravatt's level (Art. 5.) has a reflector fixed on the top of the telescope, by which the observer can see both the staff and the reflected image of the bubble at the same time, and thus can make his observation at the instant he sees the bubble in its proper position. The foregoing description of the method of taking levels is general, and applies equally to every kind of levelling operations, we shall hereafter add such additional particulars as will require attending tc, when taking levels for the formation of a section. The two first observations being com pleted, the surveyor must remove the instrument to the next station and set it up a second time, the person who held the back staff, removing it as far beyond the instrument as the instrument is beyond the staff, which has now become the back staff, and which must not be moved, except that its graduated side must be turned to the instrument. The observations may now be repeated in every respect the same as the two last described, and so on to the end of the work; excepting in cases where long sights" are taken, in which the correction for curvature and refraction must be applied, as in the example (Art. 11.); but these cases seldom occur in practice. The explanations just given, are of a more detailed character than those given in (Art. 10.), as it must be borne in mind that this work is for the instruction of those who are unacquainted with the subject, and who ought not to be led into tedious details all at once. 66 The method of taking check levels being now explained, we shall next refer to the example, in which as already stated, "the levels were taken along the public road shewn by the dotted line (Plate I.,) as being the most convenient route between the town A and B, avoiding the necessity of passing through private property; the strong black line on the plan shews where the original section was taken; the section itself is shewn above the plan, and is drawn from two scales, the one giving horizontal measure, is the same as that of the plan, that is, one inch to the mile; and the vertical scale inch to the 100 feet. From this section it appears that the crown of the bridge at A is 14 feet above the datum line DE of the section, and that the bench mark (a stone by the road side,) at B is 111 feet above the same datum; therefore the difference of level between the two places is 111-1497 feet. Now by referring to our level book, of which we have subjoined a copy, we make the difference of level to be 96.8 feet, differing from the original section only 0.2 of a foot, or 2.4 inches, a quantity that may be disregarded; the inference to be drawn from such a coincidence in the two results is, that the whole of the section between the points in question is sufficiently correct. |