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WHEN TO TAKE ANGLES.

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of the streets, that an optical square can be used) having one arm 6 ft. and the other 4 ft. long (see Fig. 211). This should be laid on the ground and adjusted until the long arm is in line with the point to which the offset is to be taken. But it is not sufficient to

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trust to such offsets to fix the corners or angles of buildings. A tie-line is necessary, as in sketch.

It is very seldom that the frontages of streets are straight or that they are of equal width. It more frequently happens that indentations of all kinds occurs as in Fig. 212, where it will be seen that in order to accurately take up the various angles and indentations a very elaborate network of triangulation is necessary, as shown by the dotted lines.

It is not sufficient at the angles formed by one street running out of another to take an offset at right angles, and from a rightangled triangle as a check. It is necessary to make an independent triangle such as Abc, Aab, Acd, Aed, Aef, Agf, agh, or sha in Fig. 213.

The diamond formed by those triangles which are hatched need not necessarily be taken, but it is quite as well to have the thing complete, especially at important points.

When the outlines of the streets have been surveyed and plotted, the surveyor should make a careful tracing of sections of the work, and then carefully walk over the route to examine every detail, so as to be satisfied that nothing has been omitted.

Then a station plan, drawn to a large scale, should be prepared and mounted, in sizes of about 18 in. square, on a board, so that the details of the houses and outbuildings may be accurately drawn to scale as the measurements proceed. A steel tape or a 10-ft. rod is the best thing for this purpose.

When to take Angles. In busy thoroughfares it is always desirable to take the angles soon after daybreak, so that the operations may not be impeded by the traffic.

BRIDGE ST.

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SURVEY OF PART OF THE TOWN

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Fig. 212.

N.B. This sketch illustrates the method of taking the projections of the various buildings in the streets.

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In measuring buildings the greatest care is necessary to see that the total length of a series of frontages is equal to the sum of the separate frontages. For this purpose the addition should always be made on the side of the field-book or upon the detail drawing, and in ink if possible.

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very serious mistakes occur by rubbing out figures which after all have proved to have been right.

If you cannot drive a peg or spike into the road, as in the case of asphalte roads, then the intersection of lines should be arranged so as to cut at some point on the curb or pavement, in order that a rail or spike may be driven in at a joint.

Use Arrows for counting.-In measuring a line along a street an arrow should be stuck in if possible, or if not, it should be left to denote the number of chains, and the leader (who should always have plenty of chalk about him) should mark with a "crow'sfoot" the end of the chain together with the number, with chalk, either upon the pavement or on the walls of the buildings.

As to Buildings.-Outhouses should be specified in the fieldbook. Churches, chapels, schools, and all public buildings should be carefully noted. Also public-houses, beer-houses, "on" and "off" licences, &c.

Lamp-posts, Gullies, &c.-The position of lamp-posts, gullies, ventilators, sluice-valves, hydrants, manholes, &c., must be taken up en route and carefully plotted on the plan.

As to Streams.-Should a street or road cross over a river or stream the full particulars thereof must be noted; and by an arrow the direction of the flow should be indicated. Or in the case of a railway crossing over or being crossed by a street, the name and particulars of the railway, together with the direction of its commencement and termination, should be ascertained and marked upon the plan. The nature of the street or road should be observed— whether gravel, macadam, granite-pitched, wood, asphalte, &c. And the pavement, whether York paving, artificial stone, asphalte, concrete, brick-on-edge, gravel, &c. The boundaries of the various parishes must be ascertained and carefully plotted, even in such a

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case as occurred to me at Hereford, where I found that the intersections of three parishes occurred in one of the bedrooms of a school-house. The parliamentary or municipal boundaries, or those of wards, must also be shown. Each road or street must be plainly marked with its name, and the thoroughfares at the outside of the survey should have written in italics the places to or whence they lead.

As to Plotting.—The survey of a town or parish should always be plotted so as to be north and south; in other words, the top of the sheet is north and the left and right sides are west and east respectively.

CHAPTER IX.

LEVELLING.

LEVELLING is the art of finding the difference between two points which are vertically at different distances from a plane parallel with the horizon. Take the ocean or a sheet of water, the calm surface of which is in a parallel plane with the horizon, then the bank or beach that is above the water-line at certain points is relatively higher in level than the water itself. Thus in Fig. 214, where a represents the impingement of the water upon

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the slopes of the stream, в is relatively higher, and c and d lower, than the horizontal line L L'.

This is a very primitive description of what levelling means, but it is nevertheless a true one.

As to the Earth's Curvature.-But there is a very important consideration in reference to this question, and that is, that the earth being spherical in form, strictly speaking two points are only truly level when they are equidistant from the centre of the earth.

Also, one place is higher than another, or out of level with it, when it is further from the centre of the earth; and a line equally distant from that centre, in all its points, is called the line of true level. Hence, because the earth is round, that line must be a curve, and make a part of the earth's circumference, or at least be parallel to it and concentrical with it, as the line P F D B C E Q (Fig. 215), which has all its points equally distant from a, the centre of the earth, considering it as a perfect globe.

But the line of sight F'D' B C'E', given by the operation of levels, is a tangent or right line perpendicular to the semi-diameter of the earth at the point of contact B, rising always higher above

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