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The form of book usually adopted is, as shown, a record of outdoor entries, headed with a description of the line upon which the section is taken and the date. The first column upon each page is intended for any general remarks, the next LEVEL column to give the angles DCE and E C F, D B E and E B F, DAE and E A F. In the third column the description of the points upon the shore D, E, and F is stated. Then the time, and the soundings are entered under their heading, while the last column entitled "R. S." contains the entries of the reduced soundings, being the heights above a given datum.
In the case of a tidal current survey the floats used may consist of a thin galvanized sheet iron cylinder about 12 in. to 15 in. diameter, closed at both ends and thoroughly watertight, provided at the top with a rope twist and a bung hole stopped with a cork carrying a small flag, while underneath are attached 3 legs made of -in. iron meeting in a ring. (See sketch.) A strong cord marked in fathoms passes from the rope twist through the ring to a wicker basket carrying about 50 lbs. of ballast. The cord connected between the bottom ring and the rope twist passes outside the cylinder, which tends to twist it slightly out of the vertical, but this is unimportant as the plotting scale is so small a ratio,
The floats should not be set deeper than 6 ft. from the bottom. It is well to start at 9 ft. from the bed of the river and to adopt a clearance of not less than 6 feet or more than 12 ft. The depth in the course of the float can be tested by the sounding line in the boat, and the depth of the basket altered accordingly so as to avoid touching the bottom. If the basket touches in a strong current, the float will go under and perhaps be lost. To prevent risk of this, the surveyor should coil the spare end of rope neatly on the top of the drum and fasten a cork to the extreme end of it; this may float and so allow the drum to be recovered.
To obtain the surface velocity, a slice of turnip may be thrown in close to the float and its velocity relative to the float noted by the surveyor. Each float is attended by a small rowing boat carrying two boatmen and a surveyor, one man manages the boat and keeps it near the float, the other attends to the soundings and takes the depth of the basket under his care, altering it as the soundings require. The surveyor takes all angles for fixing the position of the float and books all observations of sounding, etc. The field book may be kept somewhat as follows:
Time Object. Angle. Object. Sounding. Remarks.
Observations of the position of a float can be taken with advantage every fifteen minutes, commencing at the hour, and the exact time of passing four or five well defined lines running across the course taken by the float should be recorded. The depth at which the basket hangs should be noted, and the observations continued till the turn of the tide, or until some pre-arranged time or point is attained. A sketch of the principal landmarks and of each
of the buoys should be made in the field book, as without this, the record of all observations would be vague.
The apparatus required on each boat would be as follows: (1) A nautical sextant; (2) A watch, preferably one showing seconds; (3) A pair of field-glasses; (4) A chart of the water to be traversed; (5) A pocket compass for safety in case of fog; (6) A sounding lead line; and (7) A few turnips.
The lead, or sounding line, employed for nautical purposes consists of cord made of material combining the qualities of toughness with flexibility, and is usually marked to furnish fathoms and to read to quarter fathoms. Some lines are, however, marked to lengths of five feet and read to feet and tenths of a foot, with the aid of a foot rule or staff, which are occasionally of service for engineering surveys. The sounding line is loaded with a conical lead weight which may be armed with tallow in the hollow of its base, when it is required to bring up a specimen of the soil at the site; but on hard material, a jagged plunger should be employed. The lead line or sounding cord needs to be periodically tested with a standard measure during the progress of a long survey, whenever accuracy of depth needs to be very carefully recorded; it is advisable to test it daily before use. The nature of the bottom should be frequently noted, especially marking any sudden changes that may occur in the depth, as such variations in soundings usually indicate the proximity of neighbouring shoals. When taking soundings the surveyor should always try to steer in a straight line, the ends of which line should be fixed in position for indication on the plan, and the surveyor should endeavour to keep two well-defined objects in view as one point upon a continuation of this line, one near and one as far off as possible. The writer says “endeavour” to do so, because it will be found impossible with a tidal stream and wind to keep accurately upon the line, and the angles taken by the sextant will then indicate the exact position of sounding right or left of such line, but the nearer thereto the better for the plan. Soundings should not be taken during very high tides, and should be made during neap in preference to spring-tides, and in ebb in preference to flood-tides. Soundings to be taken in
flood-tides, especially during springs, should not be made till within an hour of high-water, but the ebb-tide will, as a rule, be found more steady than the flood-tide. The highwater can be timed from a mean of the time of continuance of the highest reading upon the tide gauge.
The starting position for these investigations is primarily fixed in the same manner as all other positions in a nautical survey, that is by two angles on three known fixed points, but being once found, two lines inclined as nearly as possible at right angles are obtained; the approximate position of the boat can thus be discovered at any future day without any observation with the sextant.
In a tidal navigation survey that was upon one occasion being made at the mouth of the Thames it was found that a current running at 1 to 2 miles per hour was not sufficient to sink the float when the basket grounded. The spare rope, with the cork at its extremity, will not remain on the top of the float unless the sea is very smooth.
The small flag, shown in the diagram (page 230), on the float is not needed on all occasions, and is liable to "bob " into the boatman's face when altering the depth of the basket.
The sextant remains in a much better condition if it is kept in the box when not actually in use, and it is frequently advisable to use the 66 dummy" in preference to the telescope, as such a small field is covered by the latter.
The surveyor should always take the more indistinct of two angles first, and look "directly " at the most indistinct object-if this is on the observer's right-hand side he holds the sextant upside down. Also when taking angles, he should have the boat's nose kept straight on to the waves, as less rocking takes place in that position. In fogs, and when no better objects are visible, ships at anchor may be made use of, and their position fixed later; but, as a rule, distant points are better than points close at hand, because the angles do not change so rapidly. At the same time, they are not so easily seen under varying conditions of atmosphere.
PLANS FOR DEPOSIT.
IN the instructions given by the Commissioners of the Copyhold Inclosure and Tithe Office for the preparation of first-class plans, it was provided that the Commissioners did not pledge themselves to seal a plan to which any of the following objections applied without testing it upon the ground:-(1) Wher there was any reason to distrust the authenticity or integrity of the Survey; (2) where the means afforded were insufficient to prove the accuracy of the work in all its details; (3) where the plan did not agree with the field-books; (4) where the field-books had been kept in common or metallic pencil; (5) where erasures had been made in the field-books; (6) where alterations had been made in the field-books without a satisfactory explanation being afforded; (7) where the offsets exceeded a chain in length.
The Commissioners appointed the surveyor to be employed when required for testing the plans upon the ground, and required that a sum of money sufficient to cover the expenses should be lodged in their hands before the testing was commenced.
The lines which the Commissioners required to have measured on the ground for testing the accuracy of the plan were defined as three lines in the form of a well-shaped triangle, with a proof line from one of the angles to the opposite side.
The scale (3 chains to 1 inch) employed is explained on page 128.
In measuring the testing lines, all intersections of fences were to be noted, and offsets taken within the ordinary limits of a chain's length.
Distances, measured along the fences joined, were to be