Fig. 196. N will give a similar result, as I have shown in a preceding illustration. Now let me guard against any possible misinterpretation of my meaning in the foregoing paragraphs. There are cases, as in Fig. 197, where it is quite unnecessary to take more than the six angles, A, B, C, D, E, and F, which govern the lines which absolutely affect the external boundaries of the estate, such as 1, 2, 3, 4, 7, and 8. The truncated cone formed by lines 1, 2, 3, and 4 should give by the sum of the angles A, B, E, and F 360 deg., whilst the angles C and D serve to determine the exact position of a portion of line 3 and line 7. Line 5, by reason of each of its extremities being fixed by the chainage on lines 2 and 4, should by its length be an additional check of the accuracy of the survey, whilst it serves to pick up the fence which runs alongside it. The same applies to line 6, whilst if the angles c and D and the lines 8 and 7 have been accurately taken and plotted, then line 8 should exactly fit in at their extremities. By reference to Plate USEFUL HINTS. E 153 No. I. it will be seen that a portion of my ground at Wimbledon Park is here delineated to illustrate the method of testing a chain survey. The estate, bounded on the east and south by a wood, on the west by roads, and the north by a plantation, has been surveyed by chain only on the lines 1, 2, 3, 4, 5, 6, 7, and 8 with the various check-lines as shown. Now, having thus made an accurate chain survey, it was desirable to show my pupils how I should have proceeded with a theodolite, and at the same time to check the other work. The following angles were necessary: DA B, A B C, CBE, B C D, JHD, and CDA, by means B Fig. 197. of which it was shown that the tie-lines DB, α A, E е, CG, and нh were obviated. As under the head of "Traversing" I shall have to deal with that part of this survey which has reference to the roads in the wood, I shall not at the present say anything about them. I have reproduced the field-book in connection with this survey, which will better illustrate its modus operandi. A few brief hints as to the practical part of theodolite work will form a useful conclusion of this chapter. Hints on the Use of the Theodolite.-1. It is of little use attempting to use the theodolite on a foggy, rainy, or windy day. I need not dilate on my reasons in the first-mentioned case; but in the second, the wet gets into the glasses, and the constant necessity to take them out and wipe them is not only a source of delay but a very great tax on patience; and with regard to wind, not only does it affect the steadiness of the telescope, but the chief difficulty is to keep the plumb-bob from swaying about, and unless it is perfectly plumb over the nail or cross-cut the accuracy of the observations will be impaired. 2. Before planting the instrument see that the point of the plumb-bob is exactly over the point of intersection of the line. 3. Always plant the legs of your instrument firmly in the ground as nearly level as your judgment directs. Don't force all three legs in at once by pressing from the apex, but take each leg separately, and with both hands press it into the ground. 4. Having "planted" the instrument, before you proceed to level it, take care to clamp the upper plate to the lower one at zero. 5. Now level the instrument by means of the parallel screws, having previously attended to the adjustments for collimaton, parallax, &c (referred to in Chapter III.). B 6. Now direct the telescope in direction of the extremity of the first line which forms the angle as в (Fig. 198), and when as near upon the point as is possible, clamp the lower plate and bring it exactly to allow the crosswires to intersect the point в by means of the lower tangent or slow motion. NOTE.-Do not on any account touch any other than the lower clamp and tangent screws in this operation. INSTRUMENT Fig. 198. 7. Now (having entirely done with the lower clamp and tangent screws) unclamp the upper plate and gently turn the telescope in direction of c, then clamp it at as near the point as possible, and with the upper tangent or slow-motion screw bring the cross-wires until they exactly intersect the point c. 8. Now proceed to read the number of degrees and subdivisions of degrees on the lower plate, and the number of minutes and subdivisions in the vernier. 9. Always take the lowest point of a rod, and preferably the point of it, or an arrow held upon the nail or cross-cut in the peg. In the case of a church steeple it is advisable to take the apex. * 10. The observer should not talk or be listening to conversation during instrumental observations, as the distraction of his attention often leads to serious mistakes. * Chesterfield church excepted. CHAPTER VII. TRAVERSING. WHILST Surveying proper is entirely dependent upon a system of triangles or other figures, whose sides must be accurately measured, and whose relative points of intersection must be tied in with the greatest care, traversing may be termed a method of following the meandering of any irregular figure, whose sides shall be determined by angular observation. Traversing with Chain.-Traversing may be accomplished with a chain only, but this mode of proceeding is open to great objection, as inaccuracies may find their way into the work itself, and there is no real security for its accuracy. I illustrate by Fig. 199 the general principles of a chain traverse, and I think it will be manifest to those who have read the preceding chapters that little or no dependence should be placed upon the relative positions of lines to each other, which rely solely upon the measurement of a short length at the extremities of lines. Take the lines A B, B C, C D, D E, and E F (Fig. 199), whose directions are entirely dependent upon the care with which the triangles a b в, c C D, D d e, and Eƒg are taken, and not only as affecting the measurement upon the ground, but more particularly the after operation of plotting; for, unlike a chain survey of a series of triangles and check-lines, there is nothing in a chain traverse to guarantee the accuracy of the work. Upon fairly level ground, in the enforced absence of instruments, it may be admissible to ascertain the relative positions of diverging lines by some such method, to do which even I should strongly advise the use of an optical square to establish the triangles, which, wherever practicable, should be right; but in undulating ground I do not hesitate to say that chain traversing is inadmissible. Traversing by Included Angles.Traversing may also be performed by A taking the included angles A B C, B C D, C D E, and D E F (Fig. 199) either with a box-sextant or, preferably, a theodolite. These angles having been accurately ob a B Fig. 199. e E X served, and the lengths, A B, B C, C D, D E, and E F carefully |