In preparing this little book the author has not aimed at imparting information to the practiced draughtsman, and if any such should take it up and peruse it, they might consider it trite or commonplace. On the other hand, there are a large number of aspiring young men in schools, colleges, and workshops who desire to obtain entrance into drawing offices, but are perhaps in considerable ignorance of the work carried on in such places, or of what will be required from them when there, and also of the instruments and appliances which they will need. It is to these that the author more particularly desires to s address himself, and if, after they have read this book, they find that they have derived any pleasure and profit from it, he will feel that his aim has been accomplished.


CHAPTER VI - BRANCHES OF SCIENTIFIC KNOWLEDGE ADVISABLE TO BE STUDIED BY DRAUGHTSMEN

For those likely to be engaged upon general engineering work, in the first place they cannot be too well up in arithmetic, and they should pay special attention to decimals and decimal fractions, as nearly all calculations are worked by these rules, as we will more particularly point out presently. If they are acquainted with algebra so much the better; but this is not absolutely necessary, as when there are extra abstruse calculations to be made they are generally referred to one or two men who may happen to be well up in that line in fact, some offices keep a calculator, who is expected to solve problems as they arise. It does not always appear that the best theoretical men make the best draughtsmen. Many of the best and most useful men have been obtained by taking into the office intelligent young fellows who have served their time practically in the shops, including experience in the pattern shop if possible, and who have studied in the evenings at a good technical school or mechanics institution to perfect themselves in arithmetic, drawing, and different branches of science, especially mechanics and steam, and in drawing plane and solid geometry development of surfaces, and projection of shadows. With regard to solid geometry, we do not mean to say that a man need know as much even as is expected of him to pass a second-grade advanced examination, but that he should work more on the lines of what is called orthographic projection that is, take some simple geometrical figure, such as a cube or a hexagonal pyramid, or a mechanical figure, such as a nut or a wheel, and draw it in all possible positions, as shown in fig. 69.

By mastering the principles involved in the above, and similar examples, the student will render himself able to project any work in any mechanical or other drawing that may be given him to do.

The development of surfaces comes in very useful sometimes in boiler work, when it is requisite to know the proper size and shape of a plate in the flat which will work up to a certain curved form, such as plate A in the hemispherical end of a boiler (see fig. 70), or in a Chip's buoy, &c.

Projection of shadows is a very useful study, as it enables one to correctly shade up a finished drawing, but it is a study much neglected as a rule, and the consequence is that when a drawing has to be shaded up it is often done in a very unsatisfactory way, Even if only a little shading is required, that little looks much better when put on in the right way, and the man who has studied the subject can often tell at a glance how and where to place the shadows without troubling to mark them out, first, geometrically. For instance, if we wish to shade up a round rod, by drawing a circle to represent a plan of the rod and two lines at an angle of 45, one to the centre and one tangential to the rod, we find at once the position of the highest light and the deepest shade upon the rod, see fig. 71, and work accordingly.

Also, suppose we wish to cast the shadow of a blocking course, &c., on the elevation of a house. By making a profile view of the moulding, and drawing lines at 45 from each projection on to the next face below, we at once find the depth of the shadow (see fig. 72).

Referring to the sciences which a draughtsman should study, we may safely say that mechanics, both theoretical and applied, is one of the most important of them. A man is constantly confronted with problems in this science, whether he be designing engines, cranes, tools, or girders. If, for instance, he is designing a crane to lift a certain load, say, a lo-ton hand crane, he must be able to calculate the transverse strain on the crane post, the tension on the tie rods, and the compression upon the jib. He must also be able to so arrange the gearing that the power applied, say, by four men at the handles exerting 15 lbs. each, will lift the required load of 10 tons.

The science of steam is also extremely useful, especially to men engaged upon engine and boiler work. If a man were engaged in designing a compound engine he should be prepared to say what the steam pressures would be at the end of the stroke in the high pressure cylinder; in the receiver, if there was one, between the high-pressure and the low-pressure cylinders; and on the entrance to and exit from the low-pressure cylinder, and he should be able to construct an approximately correct indicator-diagram for the two cylinders. Also he would require to so proportion the cylinders that they should, as nearly as possible, each perform the same amount of work.

Last, but not least, returning to the question of calculations, nearly all these can be done by decimals; for instance, it is often necessary to work out the weight of machinery, boilers, &c., from drawings, in which case we should proceed to find the cubic contents of the work as follows: Take the dimensions of each piece, or portion of a piece, if necessary, in feet and inches, and then turn them all into inches and decimals of an inch by means of a table of decimal equivalents; then by multiplying these together we should obtain the number of cubic inches in the piece. Suppose the article is cast iron.