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Descriptive geometry for woodworkers & metal workers

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INTRODUCTION. The object of this work is to place before the practical mechanic simple illustrations of the first principles in Geometry and the manner of making practical use of them, and to give him a proper understanding of the "square," and to furnish an easy system of lines for the framing of different classes of work and for cutting all manner of joints.

In doing this the author has taken the first principle in Geometry (the Point) and applied it to the center of the circle as a place of beginning, and relies upon it for proof of every demonstration made, as from it radiate all lines of whatsoever kind made use of by the practical mechanic. The Blade of the Oblong Square is made to represent the Horizon of the Circle and the Tongue a Tangent to the Circle.

It will be seen by observing the first illustrations in this work, that, while the apprentice or common mechanic, may be able to make use of the "square," and perform such work as may be designed for him, under instructions, he cannot hope to fit himself as a Master Mechanic without a thorough knowledge of the use of the compass and principles contained in the circle.

It is a further object of this work to enable the mechanic to understand and properly execute the plans of the architect or designer, and in the absence of such person, to be able to proportion and set forth his own work. It is a known fact that the mechanic often blames the architect with incompleteness in details and error in minor items of the work, and it is often the case.

But we, as practical workmen, should take into consideration the difference between the principles involved. The architect must first construct his work in his mind; he must not only be able to see the beginning or ground plan, but everything that is to be placed thereon, as well as the general outline and design. He must have a thorough knowledge of the specific gravity and tensile strength of every component part of the structure; in doing so he may sometimes lose sight of minor details.

The mechanic, before commencing to execute the work, has the general plans and specifications, conveying the ideas of the architect, before him, and In case of discrepancies in minor details should be able to supply them in proper manner to carry out the ideas of the architect.

There are many times that the "journeyman" has found himself suddenly thrown out of employment, or has failed to receive an advance in wages from the fact that he was uninformed as to some of the important principles of his trade.

PLATE I. At the top of this plate will be found the first principles in Geometry as made use of in this work.
1st. A point, or place of beginning, that which has place or position, but not magnitude.
2d. A line, that which has length without breadth or thickness.
3d. A broken line, made up ofstraight lines not lying in the same direction.
4th. A curve line which changes its direction at every point.

When the word line is used it is supposed to designate a straight line, and the word curve, a curved line. An angle is the portion of a plane included between two straight lines which meet at a common point.