736
THEORY AND PRACTICE OF ENGINEERING.
Book II.
Level line is that traced out by the instrument madeuse of by bricklayers and other artisans; the face gl islevel when the plummet hanging at e falls perpendicularover a line traced to^ which is set out at right angleswith it; the line of true level-is, however, a curved line,which is at all points equally distant from the centreof the earth ; for example, in a length of 5000 feet,an allowance must be made of nearly six-tenths ofa foot, to reduce the line levelled to its true level,
The plane of the sensible horizon is indicated in twomanners, first by the direction of the plummet or plumbline, to which it is perpendicular, and by the surface ofa fluid at rest: levels are therefore formed either bymeans of the plummet line, or by the use of a fluid appliedin an instrument made to contain it.
The Gunner's Level is a triangle with a scale, on whichthe plumb line falls; by which arrangement the inclin-ation of a straight line to the horizon can be measured.
Tlie plummet hangs from the point where the two equallegs of the level join at right angles, and this plays overa quadrant that is divided into twice forty-five degreesfrom the middle; so that when the plane on which theends or legs of the level rest is horizontal, the thread of theplummet falls over zero; when it falls over any otherpoint, the degree marked on the scale indicates the in-clination of tlie line to the horizon.
Diagonal line , as that in the figure A BCD, whichis drawn from A to C, cutting it into two parts, anddividing the rhombus into equal portions.
Master Line is a term given to that which is set out orboned through a country or field to be mapped, (thislatter technical expression being possibly derived fromthe French word borner f to limit or confine withinbounds,) and from which spring a number of triangles orother figures. I F is a base or master line, being thelongest which can be traced in the plot E F G III.
The use of a map is to exhibit the boundaries of acountry, and the relative position of the several parts,in reference to their just and proper proportions; thismay be done very accurately on a globe, but without thesame .spherical surface; it is not possible to representany considerable area in such a manner that the dis-tance of places shall retain the same proportions whichthey have on the globe; for small maps, or the plansof an estate, lines may be boned through for the con-struction of angles or other figures without difficulty.
Heights. — It is highly important that we should havethe means of determining accurately the relative altitudesof points on the earth’s surface : when it is required toascertain the height of one point or station relatively toanother, and also the relative heights of a number ofpoints above a common horizontal plane, as for tracingthe line of a railway, levelling is practised.
The height of a figure is a perpendicular let fall fromits highest point, as QTin the triangle QRS, or theline a b in the inclined pyramid c.
Line of heights is that which descends from the top tothe bottom of a building; O P, for instance, is such aline, from whence the observer, placed with the instru- Fig. 625.
ment Y M, is enabled to mark out at N, or any other point, a determined distance ormeasurement.
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Scales are right lines of any length,divided into equal parts; the scale A B,for instance, is set out or divided into50 feet, and one portion attached sub-divided into feet; the other is marked Fig. 626.
with 500, each portion containing 100, and this again subdivided into tens.
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