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true, that is square, for there is seldom a drawing board that will be found exactly square all round.
In entering upon the subject of mechanical drawing, whether it is such
tical worth to the plumber, as far as laying out his own work is concerned.
To show the fundamental principle of projection, let us consider the object which we show in Fig. 4, an ordinary pyramid.
Now suppose we consider this object surrounded by transparent surfaces, glass plates for instance, as shown in Fig. 5. We will suppose that the object is viewed from three different directions, from the front, from the side, and from the top. If we consider that the rays of light from each point, as these three views are taken, reach the eye at right angles to the respective glass plates, and draw on those plates the view of the object as it appears to us, we shall have what is known in mechanical drawing as
Bow Compasses actual
drawing as the plumber, the machinist, or the architect would need, it is necessary to understand the principles of projection which in reality underlies the whole subject. All working drawings, which show several views of the same object, such as are used in every machine shop, are worked out by projection, and in this class of work it is applied much more extensively than on the work in which we are interested.
With us, it will not be necessary to take up any but the most elementary principles, for although valuable to any man, the more advanced principles of projection would not be of much prac
Fig. 3 the top view or plan, and the front and side views, or elevations, as they are commonly called. In other words, the eye is supposed to sight the object at right angles at every point at one time.
three views of this object are to be obtained, that is, the top, front and side
will be readily seen that these three views give every dimension that would be required in making the given object from drawings, that is, height, width and thick
In fact, the front and top views without the side view give all that is required.
In mechanical drawing, when the object is complicated, it often happens that three views must be given to thoroughly depict the work, but in plumbing, usually only the top view, or plan, and one elevation is required.
At this point it is well to state that many of our readers will no doubt have difficulty in thoroughly understanding what we have written on projection. To these we would say that even though at the outset the subject is not clear, it will become plainer as the subject advances, and that it should not be an obstacle to going on, for we shall soon deal with the subject in a way that will appeal to the plumber from a practical and not from a technical standpoint, such as the opening of a matter of this kind must be.
N obtaining the views of an object,
that is, the projections, as they are called in mechanical drawing, such
as Fig. 7 shows in the preceding article, it is not necessary that the object be placed in any particular position when the views are taken. In ninety-nine cases out of a hundred, however, the object is supposed to be placed in the position from which the views can be most easily obtained, that is, directly facing the observer.
Suppose we consider, for instance, a short length of steel rod having six faces.
If the piece is placed so that it is squarely in front of the person making the drawing, with a face fully exposed, the front and top views would be such as
Fig. 8 shows, but if placed in some odd position, such views as Fig. 9 shows will have to be made.
Either set of views shows the dimensions of the object equally well, but Fig. 8 is preferable, because simpler. It will be noticed that in which ever position the object is placed, the two views or more that are taken must be consistent. It would not do to combine the front view of Fig. 8 with the top view of Fig. 9.
With these explanations we shall leave the subject of projection as far as its technical points are concerned, though we may have to allude to it occasionally. In writing this series, we thoroughly appreciate the fact that very few of our readers in all probability, have ever taken up the