limitations, Types, Description of the " Single Scale " Method —its essential defect, "Half Scale" Projection—its dangers, suitability for school demonstrations. The "Official " Method. Theory of Oblique Projection illustrated. Examples—cubes and prisms. A Trussed Partition—details of constructs >n. A New Method—Diminished oblique projection, its advantages. An Oblique Scale—how to construct it. A Ventilating Grating --details of joints. Method of constructing Pentagons. Projection of a pentagonal Prism. Shuttering and Forms in Reinforced Concrete Work. The Carpenter's last resource. Material to use. Size of Forms. How to construct them. Cleats and Props. Method of Drawing
This method of drawing is probably the easiest to learn and the simplest to use, for illustrative purposes, of any, and it is to be regretted that, except in the case of objects with a fairly simple outline, it cannot be used for " work shop drawings "—i.e. drawings from which the sizes can be transferred directly to the material. The reason of this will presently be obvious. It has already been stated in Chapter I. that there are three varieties of this description of projection in general use, and the author ventures to offer a fourth in the examples on page 107, which, though limited in its scope, gives, he thinks, a somewhat improved effect to the drawings for which it is suitable.
In the first method of oblique projection, the chief face or elevation of the object is drawn, as in orthographic projection, upon a plane parallel to the observer, and the receding surfaces are drawn at any convenient angle, but always in parallel pairs; for this reason the method has been somewhat loosely described as " parallel perspective."
The rear face is made parallel to the front one. Figs, i and 2, page 101, are the orthographic projections of a cube introduced for the sake of comparison, and Figs. 3, 4 and 5 are oblique projections of the solid at different angles, and the methods of producing them will be so obvious as to need no description. It will be noted that all of the retiring projectors are at angles other than right angles, hence the term " oblique."
A person to whom this method of projection is unfamiliar will doubtless conclude from the appearance of Figs. 3, 4 and 5 that the oblique side is shown much longer than the front. Of course in a " cube " all the sides are alike, as shown in Figs. 1 and 2, and if these drawings are tested with compasses they will be found so. This false appearance is the essential defect of the method ; and that next to be described is devised to counteract it.
Half-Scale Oblique Projection.—In this method, which is shown in Figs. 6 and 8, the front elevation is drawn as before, either full size or to some definite scale, but the measurements made upon the oblique projectors are to half full size or half scale, as the case may be. All lines which are parallel to the front, however, are kept to the same scale throughout. This method undoubtedly gives a more correct impression of the solid than the first method, but the introduction of the second scale renders extreme care on the part of the user necessary, to avoid errors in working to the measurements.
Oblique projection drawing is much used in schools and at classes for manual training, as it lends itself readily to blackboard demonstration purposes, anil, in or about 1896, the chief authority on the latter subject, the City and Guilds of London Institute, attempted to place the method upon a scientific basis. They published, in their " programme " of that date, a method and theory that would be accepted at their examinations for manual training teachers' certificates; the method is shown in Fig. 7, illustrating the theory of oblique projection. The object to be
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