I. Most of the illustrations presented with the following chapters are in the form of Mechanical Drawings. To the novice, these may appear confusing; but careful attention to some of the principles underlying their construction will enable him readily to interpret their meaning.

A mechanical drawing, as distinguished from a perspective drawing, or picture, instead of giving all the characteristics of an object at a glance, presents them in detail, giving in one view one set of elements, in another view another set of elements, and so on, until the form of the object is accurately defined.

For example, Fig. I is a perspective view of an object which is represented mechanically by Fig. 2. By Fig. 1 it will at once be seen that the object represented is a cylinder. In Fig. 2 there is first presented a plan, showing that the object is cylindrical; and, secondly, an elevation, showing the height of the cylinder. From the combination of these two views, the solid may be as easily imagined as from Fig. 1, and the knowledge obtained of it is much more definite.

A perspective view of an object is that which is had by looking from some one point, as A, Fig. 3, while a view represented by a mechanical drawing supposes the observer to be looking from an infinite number of points, and always in parallel lines, as indicated by A, Fig. 4.

Fig. 1

Fig. 2

Plan.

Elevation.

2. A Plan of any object represents it as it would appear if, standing on its natural base, it were looked A down upon vertically, as indicated by the arrows A, Fig. 5. If the object, as a rectangular block, has no fixed base, any one of its faces may be taken as such.

Fig. 3

Fig. 4

Fig. 5

3. An Elevation of any object represents it as it would appear if, standing on its natural base, it were looked upon in a horizontal direction, as indicated by arrows B, Fig. 5.

The elevation is always at right angles to the plan. There may be several elevations of the same object, each differing from the others as the point of observation changes. For example, the plan and elevation of the object represented by Fig. 6, are usually made as shown by Fig. 7, but they may be made as shown by Fig. 8 or Fig. 9.

Fig. 6

Fig. 7

These angular views, indeed, cannot be avoided when the form they represent is so complicated that its faces are neither parallel, nor at right angles to each other. Fig. 10 is a perspective view of an object which is represented mechanically by Fig. II. It is evident that if one face of A is shown in the elevation, two faces of B will appear; if one face of B is shown, two of A will appear.

In the representation of simple objects, the plan is in some cases omitted, and two elevations employed. These may be designated as side elevation and end elevation, which terms signify an elevation of a side and an elevation of an end. For example, if we consider the surface A the base of Fig. 6, a side elevation would be equivalent to the elevation Fig. 7, and the end elevation would become equivalent to the plan of the same figure.