848
THEORY ANI) PRACTICE OF ENGINEERING.
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all the distances measured on the station line, at which any mark, offset, or other noticeis made: in the right hand column are placed all the measurements of the offsets in thatdirection, and in the left hand column those on that side of the station line. The middlecolumn represents the station line, and whenever it passes a road or boundary, it mustbe marked obliquely to denote this deviation. The entries in the field-book are usuallycommenced at the bottom of the page for convenience, the surveyor keeping his face inthe direction of the distant station: wherever fences or other objects, as rivers or streams,are crossed, they must be sketched in the field-book in as accurate a manner as the time willpermit; by this much subsequent labour is saved. On commencing the measurementalong the station line, the letter corresponding to the starting point is placed at the bottomof the middle column of the field-book, and on each side is written the letters whencethe measurement is taken, and at every new distance this is again to be observed.
When the whole of the measurements have been made and entered in the field book, thecontents may be ascertained by computing the areas which are enclosed in the measurementof the respective triangles ; but it will be first necessary to reduce all lines measured oversteep hills to a horizontal plane ; should the inclined plane or slope not be very steep, thedifference may be rectified by holding the chain horizontal whilst measuring, which may bejudged of by the eye ; or if the slope be steep, half the length of the chain may be used: whenthe angle of inclination is 4°, an allowance of 1 in 15 is made, 6° 1 in in 7° I in 8, in 10°1 in C, and in 20° 1 in 2^, or 6 feet for every 100 feet; this, however, may be readily ascer-tained by careful observation. The angles of inclination should always be observed whereperfect accuracy is required, and the proper deductions made when the work is laid downor mapped.
Parish surveying. —Where it is required to obtain the area of the whole by some othermeans than that of adding together the contents of each enclosure, it seems the simplestmethod to commence by measuring two straight lines through the entire length and breadthof the parish; to connect the ends of these by other measured lines, and upon them asbase lines to construct triangles and measure the offsets. The contents of the entireparish may then be ascertained by calculation ; the lines measured to accomplish thisshould be shown on the plan when finished. These main lines should pass over the mostremarkable objects, as the church, the mill, the manor house, and their extremitiesshould be marked by a stone or permanent boundary, that may be referred to on allfuture occasions. This boundary should be shown on the plan by a dotted line, andwhen a fence constitutes the boundary, the dotted line should be shown on both sidesof it: when this boundary passes through a field, the whole field should be shown.The plan should be drawn to a scale of 3 chains to 1 inch, and the north point shouldalways be at the top of the plan.
To measure the base or principal line, which should be the longest that can be obtained,the theodolite is placed at one extremity, and the angle formed by this line with themagnetic meridian must be first accurately obtained : then all the angles of the severalprominent objects ; at this spot a pole must be placed perpendicularly, and proceedingalong the line, the roads, rivers, &c., must be noted as they are crossed, and all convenientoffsets should be taken. Poles should be set up at all the prominent stations, which serveto guide the measurement and insure its being in a straight line : these must be constantlyboned, or the true course will be departed from.
Where objects occur which are not accessible, angles must be taken with the theodolite,either to the right or left of the line, exactly at 00°, and then measured out to any lengthuntil clear of the obstruction : another angle of 60° must be taken and measured, thesame distance as before; these forming two sides and angles of an equiiatera! triangle,the remaining angle and side will be the same, and the distance, if measured through,will be found to agree.
After measuring up to the other extremity of the base line, the theodolite is placed uponit, and the angle of one of the side lines taken with considerable accuracy ; these side linesmust then be measured, and the theodolite placed at their extremities, so as to measurethe angles made with its transverse lines: and so proceed with the respective lines and angles,till the whole is completed. From the extremities of the two principal lines, measurethe distances one from the other, or the length of these tie lines, taking at the same timetheir angles very accurately. These angles and tie lines form four principal stations orboundary points to the parish, and should be marked permanently. The whole of theoutline between these four stations or the natural boundary may be surveyed, and afterwardsthe portions within for the filling up. The sextant is employed usefully in taking allthe interior angles and uniting them with the main lines which run through and traversethe parish : these two instruments are now the only ones employed.
To compute the area or contents of the parish, the whole should, after it is mapped, bethrown into triangles, and each enclosure treated in a similar manner: but the mostcorrect method would be to divide it into squares of about a chain, by which means thesmall parcels would have their quantities easily ascertained.