To make a complete representation of a solid, at The three projec least three projections are necessary, the plan, eleva- tions. tion and section, but frequently many more are required, and in complex constructions there should be shown, firstly, the general views of the various parts properly assembled as a whole, either in linear or isometrical perspective or in projection, and secondly, the three or more projections of each of the parts or details, usually on a larger scale. To this latter class belong Working drawings. all "working drawings" which are always made to a scale, and to prevent any mistake which might arise from errors in the drawings, all distances and dimensions should be carefully written upon the parts to which they be marked. belong. The space intended to be embraced by the figures is indicated by dotted arrows extending outward from them, thus:

Dimensions should

Written dimensions

to be taken as au

thority in cases of

Selecting the scale

and preserving its

ratio to drawing

not correspond with the distances given by the scale, and it is impossible to determine from any doubt. source which is correct, preference should be given to the numerals, unless they are manifestly in error, and the work constructed in accordance therewith must be accepted. The scale should be so large that the least dimension may be distinctly represented. It is desirable also to construct it upon the same sheet of constant. paper with the drawing, that it may be subjected to the same variations from changes of temperature, moisture and other causes, and thus the ratio of the two remain constant. A detached scale upon a piece of paper of the same quality will be found very convenient and more accurate than the dividers, Objections to the which are liable to slip or spring back as well as to injure the drawing. For small measurements the Other scales. diagonal scale is the most accurate. Triangular box-wood scales are also very useful.

dividers.

decimal systems

Constructions in Architecture, Machinery or any Decimal and duoworks executed by artisans are usually represented when used. by the duodecimal scales, whilst those for the engineer are ex

pressed in the decimal or metric system, but the rule is not absolute, and either may be used.

zontal scales fre

The vertical and horizontal scales are not always Vertical and hori the same, as it is frequently desirable to magnify quently different. the irregularities of surface lines as in profiles or longitudinal sections of earth, either above or under water, and to represent fluctuations of various kinds, as prices of materials, pressures, &c. For such purposes "profile" paper, ruled in Description of provarious forms is published either in sheets or continuous rolls. For profiles of earthworks, the ratio of the vertical to the horizontal scale is usually as one to one hundred, but may be taken at one to fifty; one to ten or any other convenient fraction.

file papers.

Sample, (The original is in red or green.)

Cross section paper is ruled in large and small Cross section paper. squares, the ratio of the scales being as one to one. It is found very convenient for sketching and computations of sections, and is frequently bound up in book form for field work.

Samples. (The originals are in blue, green or red.)

A new style of drawing paper recently patented Isometrical paper. has been designed to aid in making isometrical drawing. It is

printed in pale colors so as not to confuse the eye, and is covered by a net-work of full and dotted lines, as per sample:

Sample. (In pale drab.)

To avoid errors in reading or making drawings, some knowledge of the effect of light is necessary. It is gen- Light. erally assumed in linear representations of objects that the par. allel rays of light enter from the upper left hand Direction of. corner at an angle of 45°, so that all objects which project above a plane surface will cast shadows upon that plane, whilst all depressions in the plane will receive shadows from their edges. The edges casting the shadows are lines of shade, Lines of shade deand should be drawn heavier than the remaining fined. lines of the drawing. By carefully observing this Application. distinction it is possible to determine at a glance whether a portion of a surface projects or recedes, for example:—

Section an AB

Illustration.

B

In the first panel the centre and the borders are Explanation. raised and in the second depressed, as shown in the sections.

A general law is also deduced from these princi- Rules for drawing ples, namely, that in drawing projections, the right shade lines. hand and bottom lines should be the heavier, and in recesses the left hand and upper lines the heavier.

To render drawings still further intelligible all Section lines, or sections of solids should be shaded by a series of hatchings. parallel lines called section lines or hatchings, drawn at regular intervals. In curved or irregular surfaces, the effect may be shown by parallel lines drawn at irregular intervals.

To represent timber and metals of various kinds, colors are sometimes used with good effect, but they are not Colors-not genergenerally required, as the specifications should in- ally required. dicate with sufficient clearness the nature of the materials.

To secure uniformity in the use of colors when desired, a