Page:The American Cyclopædia (1879) Volume VI.djvu/260

 252 DRAWING FIG. 5. tionally and decimally. The scales of small harbor charts vary from 1 : 5,000 to 1 : 60,000 ; that of charts of bays and sounds is usually 1 to 80,000, of general coast charts 1 to 400,000. In the United States engineer service the fol- lowing scales are prescribed : general plans of building, 1 : 120 ; maps of ground with hori- zontal curves, 1 : 600; topographical maps comprising 1 mile square, 1 in. to 2 ft., or 1 : 2,640; 3 miles square, 1 : 5,280; between 4 and 8 miles, 1 : 10,560; 9 miles square, 1 : 15,840 ; not exceeding 24 miles square, 1 : 31,680 ; 50 miles square, 1 : 63,360 ; 100 miles square, 1 : 126,720 ; surveys of roads and canals, 1 : 600. In the plotting of sections, as of rail- way cuttings, a horizontal or base line is drawn, on which are laid off the stations or distances at which levels have been taken ; at these points perpendiculars or ordinates are erected, and upon them are marked the heights of ground above base, and the marks are joined by straight lines. To express rock in a cut, it is generally represented by parallel inclined lines ; rivers by horizontal lines, or better colored in blue ; the depth of sounding in a mud bottom by a mass of dots. Since it would be in general impossi- ble to express the variations of the surface of the ground in the same scale as that adopted for the plan, it is usual to make the vertical scale larger than that of the horizontal lines in the proportion of 10 or 20 to 1. Topographical features are represented as effectively by the brush and water colors as by the pen. Colors are used conventionally. Thus, in the practice of the French military engineers, woods are represented by yellow gamboge with a very lit- tle indigo ; grass land green, gamboge and in- digo; cultivated land brown, lake, gamboge, and a little India ink or burnt sienna, adjoin- ing fields being slightly varied in tint ; gardens, by patches of green and brown ; uncultivated land, marbled green and light brown; brush, brambles, &c., marbled green and yellow; vineyards, purple ; sands, a light brown ; lakes and rivers, a light blue ; seas, a dark blue, with a little yellow added ; roads, brown ; hills, greenish brown. In addition to the conven- tional colors, a sort of imitation of the conven- tional signs already explained is introduced with the brush, and shadows are almost in- variably introduced. Topographical drawings receive the light, the same as architectural and mechanical drawings, from the upper left-hand corner. Hills are shaded, not as they would appear in nature, but on the conventional sys- tem of making the slopes darker in proportion to their steepness, the summit of the highest ranges being left white. Topographical draw- ings embrace but a small portion of surface and are therefore plotted directly from mea- sures ; but in geographical maps, embracing at times a great extent of country, various pro- jections are made use of to express as nearly as possible a spherical surface upon a plane. These species of projection are generally in- cluded under the head of mapping.