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 horithe 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 con- file papers. tinuous 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.

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.)

It is gen- Light.

To avoid errors in reading or making drawings, some knowledge of the effect of light is necessary. erally assumed in linear representations of objects that the parallel 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 on 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 code is hereto annexed :

Paper is also manufactured in continuous rolls of 36, 42, 54 and 59 inches in breadth and of various weights, qualities and colors. The light brown manilla is extensively used for working drawings. Hot pressed papers have a smooth Hot and cold surface; cold pressed, rough. The latter are gener- pressed. ally preferred.

drawings for con

The drawings to accompany a contract should Requirements of be so full, complete and accurate as to leave noth- tracts. ing to the imagination or discretion of the contractor that may be shown on paper. All the sheets should be carefully designated by letters or numbers for convenience of reference, should contain a title, a scale, the date when finished, and name of the person who may be consulted in cases of doubt or errors, should they arise.

In addition to the above, maps should always For maps. show a meridian and the magnetic variation when it is possible to determine it. These last requirements are frequently overlooked.

Every piece of work, however small, should be Necessity for drawrepresented by drawings, that the artisan may have ings.

a distinct idea of what is to be done.

For Government works, the law of 15th July, 1870, is very explicit. It provides that,

Law requiring plans and estimates work.

for Government

"Before any new buildings for the use of the United States are commenced the plans and full estimates therefor shall be prepared and approved by the Secretary of the Treasury, the Postmaster General and the Secretary of the Interior; and the cost of each building shall not exceed the amount of such estimate."* The above law requires not only drawings but Complete estimates estimates in full, which shall be equal to the cost of required the proposed structure. It is important therefore that the engineer or architect should have sufficient prescience based upon experience to include and provide for every item or contingency that may arise in the progress of the work, that his estimates may not fall short of the actual cost, as is too often the case. The principal items entering into estimates with the manner of working them up will be found more fully developed in the succeeding chapter.

* Revised Statutes U. S. Section 3734.