MECHANICAL DBA WING SHADOWS, ETC. ] APPLIED MECHANICS. 



791 



showing the parts in their proper dimensions, so that a 

 workman can measure and transfer them to the solid 

 material colours are usefully employed to distinguish 

 the different materials. Thus, parts tinted dark blue 

 might represent cast-iron ; while a pale blue might indi- 

 cate wrought-irou or steel ; yellow would mark brass or 

 gun-metal ; brownish-yellow, wood ; brownish-red, cop- 

 per ; and so on. When any special material is to be 

 used, its name is generally written on such part of the 

 drawing as represents it. 



CENTKAL LINES. In almost all mechanical works, 

 or at least in their details, there is necessarily symmetry, 

 or repetition of equal parts on different sides of some 

 central points. For instance, a cylinder, cone, column, 

 and Buch-like bodies, are symmetrical round their axes. 

 The teeth of a wheel are symmetrical round its centre ; 

 and so with almost all the forms suited to machinery. 



It is, therefore, most important in mechanical draw- 

 ings, to mark the central or axial lines of the different 

 parts. As these lines do not necessarily appear on the 

 material, they are generally distinguished from those 

 that mark outlines of actual form, by the use of a differ- 

 ent-coloured ink ; for instance, if the lines of the 

 drawing generally be in black, as they usually are, the 

 central lines may be ruled in red or blue. They should 

 always be fine but distinct lines, because they serve as a 

 starting-point for numerous dimensions. As an illustra- 

 tion, we may refer to Fig. 34, which we may suppose to 

 be the section of a round or cylindrical box, the centre 

 or axis of which is marked by the dotted line a. One of 

 the bolts for fixing the cover is also shown ; and as this 

 bolt is also cylindrical, it has an axis or central line 6, 

 and the position of the bolt is sufficiently indicated on 

 the section by figuring the distance of its axis from that 

 of the cylinder, as it is marked, 10, on the figure. We 

 have indicated the central lines a and b by dotted lines, 

 to distinguish them from the outlines and hatchings ; on 

 a drawing they would probably be marked in blue or red, 

 not dotted, hut plain, fine, and distinct. 



SHADOWS AND SHADES. In order to render 

 mechanical drawings more intelligible than they other- 

 wise would be, recourse is had to shadowing and shading. 

 The projections which constitute mechanical drawings 

 are not representations of the actual appearances of the 

 objects to the eye ; they are merely the traces of their 

 outlines formed by parallel lines drawn from all parts of 

 them, to certain imaginary planes or flat surfaces, which 

 are supposed to be transferred to the paper. In like 

 manner, in the shadowing and shading of mechanical 

 drawings, wo must not attempt to give the natural 

 appearance of the lights and shades risible on the 

 objects. We must concert some system of illuminating 

 their surfaces, in accordance with our system of project- 

 ing their outlines. The object of such light and shade 

 is to make up for the deficiency of one of the dimensions 

 of solid objects, depth, thickness, or distance, when we 

 represent them on paper or any flat surface. 



These outlines, however accurate, can never show more 

 than two dimensions length and breadth ; and can 

 convey no idea of parts projecting or receding from any 

 surface represented. Nor can light and shade, applied to 

 these outlines, give the other dimensions, so that they 

 can be measured ; but they can be applied with very 



Again, shading may be very serviceable in giving a 



Fig. 38. 



just idea of the form of an object seen in elevation. 

 Thus a (Fig. 39) is the bare outline elevation of a roller 



Fig. 40. 



a 



standing on one end ; but it gives no notion of the round- 

 ness of the object. 6 is the elevation shaded, and at 

 once conveying the idea of roundness. 



When the lights, shades, and shadows are all intro- 

 duced in their proper places, as in Fig. 40, the notion of 

 solid form is rendered very distinct, and a very clear 

 conception of the object represented is conveyed to the 

 mind. 



The mode in which shadows are drawn is very similar 

 to the system of projecting the outlines themselves. If 

 we suppose a body, such as a die, to stand on a table, 

 and numerous parallel rays of light, indicated by the 



Fig. 41. 



\\\\ 



Fig. 37. 



good effect to indicate which parts 

 are supposed to project or recede 

 from the general surface, and the 

 comparative amount of such super- 

 ficial variations. If we have, for 

 instance, the outline elevation of 

 a square block of some material 

 (Fig. 37), we cannot judge whether 

 the outline is intended to represent 

 a prominence or a hollow in the 

 general surface of the block ; but 

 by a little shadowing we can give the effect of either, as 

 in Fig. 38. In a we see that the inner part must project 

 beyond the outer, in order to cast a shadow upon it ; 

 while in b the outer part of the surface must project 

 beyond the inner for the same reason. 



dotted lines, to strike on it at an angle, some of them 

 would be intercepted by the die, and thus all the surface 

 of the table and the die would be illuminated, with the 

 exception of the portions from which the light is kept 

 by the interposition of the solid body. The dark surface 

 thus left on the table, is a shadow of the die ; and the 

 unilluminated side of the die is a shade on the die. It 

 appears, then, that the shadow is merely a projection of 

 the figure made on a plane, by tracing oblique parallel 

 lines from all the points in the outline of the body, to 

 that plane. 



But as this projection may, like the projection of the 

 object itself, be effected on more than one plane, and as 

 we are at liberty to suppose the object placed in light 

 coming in any convenient direction, we can select such 

 an obliquity of the rays as may at the same time furnish 

 us with most distinct shadows, and with those most 

 easily drawn. In selecting, then, the angle at which the 

 rays of light shall strike, we can assume any that is not 

 coincident with the planes of projection on which our 

 drawing is made. But as these planes are at right 

 angles, or square to one another, it seems natural to 

 select half a right angle, 45, or what is generally termed 

 by workmen a mitre-angle, as the direction of the rays. 

 Agaiii, as a line drawn at such an angle may extend 

 either from below upwards, or from the right towards 

 the left, or from a distant point towards us, we must 

 select the most suitable of these courses for giving us 

 distinct shadows. As we are generally accustomed to 

 see objects illuminated by light coming from above, it 

 is natural that we should determine on employing rays 



