is furthermore necessary to remove the instrumental parallax, and to see that the line of collimation is correct, so that when the telescope is set level, parallel to the optical axis, and to the surface of the cylindrical rings upon which it is supported, the cross wires in the diaphragm shall coincide with the axis of the supports in which the telescope rests. The distance apart, as well as the uneven nature of the surface of the ground between the points A and H, render it necessary to call upon intermediate points, such as B, C, D, E, F, and G, and to re-set up the instrument between each change of position in the level staff. The instrument is set up level between A and B ; the staff is placed upon A, and found to read 2.20. This is booked as a back-sight, because we are looking back upon a point the level of which is already known. The telescope is then revolved horizontally; the staffman is directed to move forward to B, or to a point where the observer can read as great a height upon the staff as possible, owing to the falling inclination of the ground. The staff at B is booked as a fore-sight, because the observer is here looking forward to a point the level of which he does not yet know. Having read B, he then moves the instrument forward, while the staffman is entrusted to maintain the foot of the level-staff very carefully upon the point B. To assist the staffman in so doing, he generally employs a foot-plate or iron peg and waves the staff upon it to and from the telescope, like an inverted pendulum over a vertical line, or line normal to a tangent to the earth's surface at the spot where the level staff is being held when the observer is reading the divisions. With the instrument in the new position between B and C, placed so as to read as low down as practicable upon the staff, the sight III is then read. This is booked as a back-sight, because when the level book is reduced, the level of the point B can be arrived at before the sight I'II is considered. Suppose the ground to be such that the instrument is placed upon the left hand of A, when the sights 2.20 and 12'09 are read, or upon the left hand of B, when III and 11'70 are read with the telescope pointing in the same direction, the first sights taken with the instruments would still be booked as back-sights" for the 66 reason above stated, although the observer would be looking forward, in each case, so far as direction is concerned. The position of the level staff is localised, not that of the level. The level is simply set up in any convenient position for reading the staff, and the surveyor should never hesitate to take an extra sight, off his line of section, if need be, as a fore-sight, in order to secure a firm position for the level staff while changing the position of the level. The intermediate spots upon which the staff is held between A and H can be described, if required, by letters referring to an accurate plan, from which the horizontal distances can be afterwards measured with a scale for the purposes of plotting, or the whole can be chained in the field as a continuous section. Where the ground is undulatory, and an accurate vertical section is required, it will be found necessary to take some sights between the first and last, with the level in any single position. These are called intermediate sights, and the entries are recorded in a separate column. (See pages 173-176.) In the diagram of "Flying Levels," if the back-sight at A reads 2.20, the level of the line of collimation would be 62 20, and the level of the ground at B would be found by subtracting the fore-sight 12:09 from 62.20, giving a result of 50'11. Again, adding the back-sight III to 50'11, we obtain the level of the new line of collimation between B and C, equal to 5122, and subtracting the fore-sight 1170 we obtain 3952 as the level of the ground at C. Thus, by continuously adding the back-sights and subtracting the fore-sights taken between A and H, we may obtain the reduced level of the point H as 39'42. This will be clear upon reference to the diagram (pages 166-168). Now, if instead of first adding 2.20 to 6000, and subtracting 12:09, we took the difference between 2'20 and 12.09 and subtracted this difference from 60'00, we should obtain the same result, 50'11, independently of any calculation for the level of the line of collimation. It will be seen that this is exactly the usual method adopted in a level book, in which columns are provided for the amounts of rise and fall; the difference constituting a rise when the Land Surveying and Levelling, pp. 166, 167, 168. LUMNS, CIVE D. MNS, CIVE CE. C.D.E.F.C.M 60.00 20.58 39.42 NOTE THE FIRST SIGHT READ OFF THE LEVEL. THE LAST SICHT READ OFF THE LEVEL CHT TO THE SICHT BOOKED IN THE LINE 30 FELT fore-sight is less than the back-sight, as between D and C, or E and F or a fall when the fore-sight is greater than the back-sight, as between A and B, or C and D. (See page 173.) Level Book-The annexed pages (173-177) give the readings taken upon a staff, in the first place, when adjusting for collimation, and assume that the circumstances are such that the collimation is 0.06 foot out of truth, showing that correction must be made, until the staff at W reads, in this case, 8.86. They also show the readings booked in taking certain check levels. The system of flying levels is adopted for running a check level after taking a section. In the record of the longitudinal section taken on July 16, 1900, the student is recommended to copy into an actual Level Book the entries here provided, and then to reduce the levels page by page, there being four pages in this section to reduce. All readings between the first and last sights before changing the position of the instrument are entered as intermediate sights. The first sight read off the level staff is booked as a back-sight, and gives the height of the line of collimation in the instrument, above the point upon which the level staff is placed. The last sight or reading read off the level staff before changing the position of the instrument is booked as a fore-sight. The columns of distances or lengths give the position of the level staff. Horizontal distances are expressed in links, and are localised by description in the column headed Remarks, which usually occupies the whole of the right-hand side of each double page of the book. The first back-sight is preferably taken on a bench mark. The reading of each intermediate and fore-sight gives the vertical distance between the line of collimation and the points upon which the level staff is held. The surveyor observes the staff while it is being waved and books its lowest reading; he then should (if at all uncertain) look again at the staff to corroborate the observation, and if the record requires alteration, use a piece of india-rubber, which he should always have in his pocket ready for use, so as to keep his level book clearly and |