A FLAT rectangular board is first to be provided, of any convenient size, as from 18 to 30 inches, and from 16 to 24 inches broad. It may be made of fir, plane tree, or mahogany; its face must be planed smooth and flat, and the sides and ends as nearly as possible at right angles to each other- the bottom of the board and the left side should be made perfectly so; and this corner should be marked, so that the stock of the square may be always applied to the bottom and left hand side of the board. To prevent the board from casting, it is usual to pannel it on the back or on the sides.

A T square must also be provided, which by means of a thumb-screw fixed in the stock, may be made to answer either the purposes of a common square, or bevel,-the one-half of the stock being movable about the screw, and the other fixed at right angles on the blade. The blade ought to be somewhat flexible, and equal in length to the length of the board.

Besides these, there will be required a case of mathematical instruments; in the selection of which it should be observed, that the bow compass is more frequently defective than any of the other instruments. After using any of the ink feet, they should be dried; and if they do not draw properly, they ought to be sharpened and brought to an equal length in the blade, by grinding on a hone.

The colors most useful are, Indian ink, gamboge, Prussian blue, vermilion, and lake. With these, all colors necessary for drawing machinery or buildings may be made; so that, instead of purchasing a box of colors, we would advise that those for whom this book is intended should procure these cakes separately: the gamboge may be bought from an apothecary- a pennyworth will serve a lifetime. In choosing the rest, they should be rubbed against the teeth, and those which feel smoothest are of the best quality.

Hair pencils will also be necessary, made of camel's hair, and of various sizes. They ought to taper gradually to a point when wet in the mouth, and should, after being pressed against the finger, spring back.

Black-lead pencils will also be necessary. They ought not to be very soft, nor so hard that their traces cannot be easily erased by the Indian rubber. In choosing paper, that which will best suit this kind of drawing is thick, and has a hardish feel, not very smooth on the surface, yet free from knots.

The paper on which the drawing is to be made, must be chosen of a good quality and convenient size. It is then to be wet with a sponge and clean water, on the opposite side from that on which the drawing is to be made. When the paper absorbs the water, which may be seen by the wetted side becoming dim, as its surface is viewed slantwise against the light, it is to be laid on the drawing board with the wetted side next the board. About half an inch must be turned up on a straight edge all round the paper, and then fastened on the board. This is done because the paper when wet is enlarged, and the edges being fixed on the board, act as stretchers when the paper contracts by drying. To prevent the paper from contracting before the paste has been sufficiently fastened by drying, the paper is usually wet on the upper surface, to within half an inch of the paste mark. When the paper is thoroughly dried, it will be found to lie firmly and equally on the board, and is then fit for use.

If the drawing is to be made from a copy, we ought first to consider what scale it is to be drawn to. If it is to be equal in size to, or larger than the copy, a scale should be made accordingly, by which the dimensions of the several parts of the drawing are to be regulated. The diagonal scale, a simple and beautiful contrivance, will be here found of great use for the more minute divisions; and whenever the drawing is to be made to a scale of 1 inch, inch, inch to the foot, a scale should be drawn of 20 or 30 equal parts; the last of which should be subdivided into 12, and a diagonal scale formed on the same principles as the common one, but with eight parallels and 12 diagonals, to express inches and eighths of an inch. For making such scales to any proportion, the line L on the sector will be found very convenient.

Great care should be taken in the penciling, that an accurate outline be drawn, for on this much of the value of the picture will depend. The pencil marks should be distinct, yet not heavy, and the use of the rubber avoided as much as possible, as its frequent application ruffles the surface of the paper. The methods already given for constructing geometrical figures will be here found applicable, and the use of the T square, parallel ruler, etc, will suggest themselves whenever they require to be employed.

The drawing thus made of any machine or building is called a plan. Plans are of three kinds-a ground plan, or bird's-eye view, an elevation or front view, and a perspective plan.

When a view is taken of the teeth of a wheel, with the circumference towards the eye, the teeth appear to be nearer as they are removed from the middle point of the circumference opposite the eye, and it may not be out of place here to give the method of representing them on paper:-If AB be the circumference of a wheel as viewed by the eye, and it is required to represent the teeth as they appear on it, only half of the circumference can be seen in this way at one time, consequently we can only represent the half of the teeth. On AB describe a semicircle, which divide into half as many equal parts as the wheel has teeth; then from each of these points of division draw perpendiculars to the wheel AB, then will these perpendiculars mark the relative places of the teeth.

When the outline is completed in pencil, it is next to be carefully gone over with Indian ink, which is to be rubbed down with a little water, on a plate of glass or ealhernware-so as to be sufficiently fluid to flow easily out of the pen, and at the same time have a sufficient body of color. While drawing the ink lines, the measurements should be repeated, so as to correct any error that may have occurred during the penciling. The screw in the drawing pen will regulate the breadth of the strokes; which should not be alike heavy; those strokes being the heaviest which bound the dark part of the shades. Should any line be wrong drawn with the ink, it may be taken out by means of a sponge and water, which could not be done if common writing ink were employed.

In preparing for coloring it is to be observed, that a hair pencil is to be fixed at each end of a small piece of wood, made in the form of a common pencil, one of which is to be used with color, and the other with water only. If the color is to be laid on, so as to represent a flat surface, it ought to be spread on equally, and there is here no use for the water brush; but if it is to represent a curved surface, then the color is to be laid on the part intended to be shaded, and softened towards the light by washing with the water brush. In all cases it should be borne in mind, that the color ought to be laid on very thin, otherwise it will be more difficult to manage, and will never make so fine a drawing.

In colors even of the best quality, we sometimes meet with gritty particles, which it is desirable to avoid. Instead of rubbing the color on a plate with a little water, as is usual, it will be better to wet the color, and rub it on the point of the forefinger, letting the dissolved part drop off the finger on to the plate.

Mechanical Drawing And Perspective 32

In using the Indian ink, it will be found advantageous to mix it with a little blue and a small quantity of lake, which renders it much more easily wrought with, and this is the more desirable as it is the most frequently used of all the other colors in Mechanical Drawing, the shades being all made with this color.

The depth and extent of the shades will depend on various circumstances - on the figure of the object to be shaded, the position of the eye of the observer, and the direction in which the light comes, etc. The position of the eye will vary the proportionate size of any object in a picture when drawn in perspective. Thus, if a perspective view of a steam engine is given, the eye being supposed to be placed opposite the end nearest the nozzles, an inch of the nozzle rod will appear much larger than an inch of the pump rod which feeds the cistern; but if the eye is supposed to be placed opposite the other end of the engine, the reverse will be the case. But in drawing elevations and ground plans of machinery, every part of the machine is drawn to the proper scale-an inch or foot in one part of the machine, being just the same size as an inch or foot in any other part of the machine. So that by measuring the dimensions of any part of the drawing, and then applying the compass to the scale, we determine the real size of the part so measured. Whereas, if the view were given in perspective, we would be obliged to make allowance for the effect of distance, &.c.

The light is always supposed to fall on the picture at an angle of forty-five degrees, from which it follows, that the shade of any object, which is intended to rise from the plane of the picture, or appear prominent, will just be equal in length to the prominence of the object.

The shades, therefore, should be as exactly measured as any other part of the drawing, and care should be taken that they all fall in the proper direction, as the light is supposed to come from one point only.

It is frequently of great use for the mechanic to take a hasty copy of a drawing, and many methods have been given for this purpose-by machines, tracing, etc. We give the following as easy, accurate, and convenient.

Mix equal parts of turpentine and drying oil, and with a rag lay it on a sheet of good silk paper, allowing the paper to lie by for two or three days to dry, and when it is so it will be fit for use. To use it, lay it on the drawing to be copied, and the prepared paper being nearly transparent, the lines of the drawing will be seen through it, and may be easily traced with a black-lead pencil The lines on the oiled paper will be quite distinct when it is laid on white paper. Thus, if the mechanic has little time to spare, he may take a copy and lay it by to be recopied at his leisure.

Care and perseverance are the chief requisites for attaining perfection in this species of drawing. Every mechanic should know something of it, so that he may the better understand how to execute plans that may be sub-mitted to him, or make intelligible to others any invention he himself may make.