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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.)

LONGITUDINAL AND TRANSVERSE SECTIONS. 189

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

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that "a good surveyor will practise the judging of distance, and will often test his ordinary paces when walking, so that he can obtain the length of a line pretty accurately without the aid of tape or chain. It often happens that he has to cross forbidden ground over which he cannot chain a line, and this plan will be found most useful. I was once thus stepping the distance across a large field, the property of an irascible farmer, and had proceeded nearly two-thirds of the way when he confronted me with a big stick, saying, 'Hi, you. What business have you on my property? You're a spoiling of these turmits.' 'My good friend, I am doing no harm.' 'I'm no friend of yours; I am your henemy. Come, get out of this, can't yer?' 'Well, then, my good enemy, you see I have already got two-thirds of the way across this field, and it will do the turmits, as you call them, less damage if I go off them in this direction.' 'Wall, may be, it wull; but I'll see you safe off afore I goes away.' 'Thank you. Good afternoon. 196, 197, 198,' and so on until the line was complete. Sometimes, however, an opposing landowner knows what he is about much better than this simple farmer, and puts the surveyor to considerable inconvenience. Imagine the disgusting case of a friend of mine, who in taking a line of levels was constantly confronted by a couple of rustics holding up a large piece of rick-cloth stretched between two poles. Who could object? They were at liberty to stand where they liked, and hold what they liked, and the old farmer stood by and chuckled derisively At length, with a bow, the surveyor retired. The next morning the farmer and he met again some distance ahead. Ah, old fellah!' said Hodge, 'I did yer last night. I know a thing or two.' 'Yes,' quietly remarked my friend, 'but you don't know everything. I was back after dark and completed my line of levels with the aid of a lantern.' It is not an easy thing to take levels with the aid of an artificial light, but it may be done, and it is useful to know that, with a little care, it can be effected."

CHAPTER XVII.

CONTOURS.

A CONTOUR plan enables the surveyor to judge the undulatory nature of any ground, and in setting out work to run lines approximately of equal level or of uniform gradient. (See pages 194, 195.)

They are plotted upon a plan by lines representing the imaginary intersection of horizontal planes with the surface of uneven ground, and form the outline of a horizontal section or the outline of any portion of the hill made by still water rising to that height. The figures written upon the chain dotted lines in the accompanying plan, represent vertical intervals of five feet. The plan is prepared in the following manner. Suppose the altitude of the contour required to be 120 ft. above a given datum (see page 177), and that the nearest reliable bench mark is known to be at a level of 137.86 ft. above the same datum, there is a difference of 17.86 ft. between the level of the bench mark and the required contour. The instrument is set up in a convenient position and adjusted, and if, for example, the staff reads 261, this is booked as a back sight and shows the line of collimation to be at a level of (2.61 plus 137·86) 140:47 ft. above the given datum. Now the contour being 20:47 ft. below this, it is evident that the line of collimation must be lowered in position before a level staff, 14 ft. or 16 ft. long, could be read by the instrument when set up level. A fore sight for the purpose of connecting the levels is taken in such a position that the staff is read near the top, say at 12.85, and the level is re-set in a new position to read near the foot of the staff, say 183. Reducing the book we find the level of the point upon which we changed the level of the line of collimation to be 12762. The position of this point is not recorded in the level book, as it is not required

to be indicated upon the plan, but we have by this means fallen from a known level of 137.86 to a known level of 127 62, and have now only to fall 7.62 ft. lower to find the position of a point upon the required contour. Adding 7.62 to 183 we obtain 9:45 as the necessary reading upon the staff to give the height required, and this figure is then booked for reference in the intermediate column. If the staff reads less than 9:40 the staffman is told to go to lower ground, whereas if the staff reads more than 9'50, the staffman is told to come to higher ground. Except where extreme precision is required, the divisions indicating hundreds may be disregarded in reading the staff, but it is well to observe them in the Level Book for sake of accuracy in connecting the levels. So far as points of equal level are concerned, a reflecting level would serve to fix them approximately, but the most accurate work is secured by the aid of a Dumpy level as above described. Signals will have to be agreed upon by which the surveyor can communicate with the staffman when they are too far apart to be within hearing of one another. The position of the contour points in the field is fixed during the process of levelling by stout laths pointed at the ends, so as to be left temporarily in the ground at suitable distances apart. The number of laths required will entirely depend upon the intricacy of the ground and the size of the features. Laths are placed in positions determined by levelling at such points as will best define the shape of the contour, and these laths are afterwards surveyed in a similar manner to points in a fence. It is advisable only to level and lath out as much as can be surveyed before leaving the ground for the day.

Should the contour pass round the shoulder of a hill, so that the staffman in approaching the next point is out of sight of the surveyor at the instrument, or should the distance become too great for the staff to be distinctly read through the telescope, the instrument must be moved to a fresh position, and the levelling connected. Suppose, after fixing the contour laths marked A, the surveyor decides to shift the instrument, and directs the staffman, for the purpose of securing accuracy in the change, to hold the staff upon the changing peg or footplate. If the foresight reads

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