ex each side of the centre line, and in order to continue the levelling between the bench marks, they are levelled alternately thus:---C. S. No. 1 is taken from right to left, C. S. No. 2 from left to right, C. S. No. 3 from right to left, viewed in the direction of the chaining of the longitudinal. section, as indicated by the arrows upon the plan. This continuous method is found in practice to be the most satisfactory way of connecting transverse sections with a longitudinal section, as the latter can be more peditiously proceeded with, where there are numerous cross sections required, than if the cross sections are taken all in the same direction. They are, however, all plotted in a direction to read from left to right. Sometimes, at the outer end of a transverse section upon falling ground, the level staff may appear to be a little short, in which case, rather than change the position for a sight, which, though needed to complete the section, may not at this distance from the centre line seriously affect the future work, a surveyor will instruct his staff holder to raise the staff one foot or two feet, which another man could measure against a ranging-rod, and thus obtain a view of the sub-divisions of the Level staff upon the line of collimation. In the case of a river section, a cord or wire may be stretched between the two opposite banks, and horizontal distances subsequently marked thereon, after it has assumed its final sagging, by attaching short pieces of tape at every 5 or 10 feet, but it will be found more expeditious to employ a rope on floats pulled tight, and to measure the horizontal distances upon this rope. Having taken the water level very accurately, and noted the hour of the day in so doing, in case its level may be affected by tidal action, soundings are taken at the points indicated, from which a section can be plotted, giving the transverse bed of the river. The longitudinal section of the bed of the river could be subsequently made up from several cross sections, accompanied by an accurate plan. When levelling in connexion with proposed works likely to be influenced by flood levels, ordinary or extraordinary, it is necessary to gain all the information that can be acquired as to the different levels of brooks, streams and Land Surveying and Levelling, pp. 186, 187. PRESENT SURFACE OF CROUND ABOVE O.D. ECTION No 2 C.S.N°2 JUO FEET ABOVE O.D. SECTION 300 400 300 FEET rivers crossed, and to enter all the data so learnt in the page of the level book headed "Remarks." In order to approximately gauge the volume of water flowing in the channel of a stream, it is necessary to set up an artificial dam, which is usually constructed of stout planking, in which a rectangular notch is cut; for a stream, this notch will rarely exceed a foot or a foot and a half in depth; the edges of this notch should be chamfered at an angle of 45°, and be neatly lined at the crest with iron. This overfall should be strongly built, and with close joints; it must be solidly set up, so as to resist without bending, swerving, or leaking, the body of water which will accumulate behind it; it must be set up truly level, with the sharp crest facing the current of the stream, also sufficiently high out of water, so that the sill will not be "drowned," by floods, but at the same time not so much as regards streams more particularly, as to cause the adjacent fields to be inundated by the first rainfalls, for, besides other consequences, there would then be an end of the stream gauging for a time. It is often very important to ascertain the quantities arising from flood waters. Before setting up the overfall, a stout post should be firmly driven into the bed of the stream some seven or eight feet above the site of the overfall, and as soon as this latter has been well and soundly got in, the post should be cut down exactly to the level of the sill of the overfall; this should be adjusted with a spirit level; a sharp-edged rule, decimally divided (in feet), should then be fixed to the post with the zero level with the edge of the sill on the overfall; by means of this rule the depths of water flowing over the notched board will have to be measured. When from heavy rains the stream swells so that the waters approach the overfall with an appreciable velocity, it will have to be taken into account. With regard to the length of the sill of the overfall, it should not be much less than the width of the channel in which it is set up, or it will cause a wire drawing of the stream. In a wide river, the system explained under the head of Marine Surveying will probably have to be adopted. (See pages 226-233.) In the process of taking levels, an experienced surveyor will, in fine weather, endeavour to sight as much as possible towards the west in the forenoon, and towards the east in the afternoon, so as to have the sun as much as possible upon the level staff. He will stand with his back to the sun, as it is better to have the sun upon the staff than upon the object glass of the instrument. It sometimes happens in the course of running a section, that the surveyor comes upon a very abrupt difference in the level of the ground, too great to connect with the level-staff, such as a vertical rise or fall of 20 feet, more or less. The best plan to adopt in such a case, is to level round without chaining more than the line of section requires, but if the ground does not admit of such connexion of levels within a reasonable distance, then a break must be made in the series of levels and the exact height checked by means of a tape or a measuring rod. The levels of both the upper and lower points should be arrived at by levelling from distinct bench marks, in order to determine the correct difference of level accurately. As with the use of an ordinary plotting scale, the value of the second decimal expressing hundred parts can not be scaled with sufficient accuracy, especially with liability to shrinkage in the paper upon which the section is drawn, it is usual to figure upon the section the heights above the datum line of all important points. It is also usual to add the total length in miles, furlongs and chains at the end of the line of section. Horizontal scales are best drawn horizontally. Vertical scales are best drawn vertically on the paper. (See pages 182, 184.) The late Principal of the Crystal Palace Company's School of Practical Engineering at Sydenham, when addressing his students in 1876, gave them the following valuable hints: "Always keep your book clear and distinct; do not be afraid of entering too many particulars; take plenty of bench marks when you are levelling, for future reference; keep the glasses of your instruments clean, but do not fall into the error of one of my young friends who, being on a survey, telegraphed to me one morning to this effect, 'I have thoroughly cleaned out the level from end to end with my pocket handkerchief, and for the life of me I cannot see any cross hairs.'" The principal then went on to say |