7G2

THEORY AND PRACTICE OP ENGINEERING.

Book II.

Fig.

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V

cosine and the radius, hence if the radius be taken as unit, the secant is the reciprocal othe cosine. The cosecant and cosine is the complement of an angle or arc.

The sides of the triangle ABC are sines,because its sides are enclosed in the circleE F G B D, which has for its radius one of thesides A.

A tangent is a right line which falls perpen-dicularly on the end of the radius, where ittouches the circle, as H F: A H is a secant.

To trace a line or any figure on the ground,the engineer is provided with staves or rods ofvarious lengths which enable him to stationthe holder at any particular point where theobservation is required to be made ; such a staffor piquet is made use of, with various devices onthe point, to set out a straight line through anydistrict that is to be surveyed. Any one sta-tioned at either of the extremities of the lineA D could direct others at the intermediatestations F, C, H, B, E, to plant their rods in thedirect line ; which when set out is to serve asthe base or diagonal of a figure, from which thecountry around is to be mapped or levelled.

Sometimes these piquets have placed uponthem boards, either coloured or pierced withholes, or with curved tops, which enable* themto be more readily distinguished.

The staves made use of for levelling have avane which slides up and down with facility, andcan readily be lengthened ; these are accuratelydivided in their height into hundredths of a foot,and are alternately coloured black and white:the lines denoting the tenths are drawn throughthe whole breadth, and every | and ^ foot isfurther distinguished by one or two conspicuousdots or marks.

Smaller stumps or pins are made use of tomark out the line or figure after the survey iscommenced ; Q, is a form made use of for onedescription of marks, and P for another.

When the excavator commences his labour,he is generally instructed to leave witnesses of hiswork in the shape of small cones of earth, bywhich the depth of his excavation can always beobtained. B is the natural and original level ofthe man A. At every extreme height as well asdepth, these marks should be left for the engineerto make his survey when he is desirous of verify-ing the quantity of earth which has b‘»en removed.

In measuring such an excavation, it is thehole made in the ground that is to be measured,and not the earth removed, and when greatdepths are taken out, allowances in price aremade accordingly; in large excavations theworkmen are usually distributed in gangs ; onedigs, another fills, and a third wheels the earthaway : when the distance exceeds twenty yards,a fourth man is employed, a stage being con-sidered that distance. The first man, who hasto wheel the earth out of the work, generally hasan inclined plane to mount, whilst the secondruns his barrow along level ground ; when thisis the case, the stage or run is limited to a lessdistance, and when the level is an inclined plane,then the run is extended to twenty-five yards.

There is a considerable advantage obtained individing the moving of earth into separate stages,but the engineer who has the direction of the Fig .

Fig. 791.

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Fig. 792.