700 



APPLIED MECHANICS. [SIKCHAKICAL DIUWISO H\T. IIINO. 



the various intersections of one curved surface with 

 another. We shall give one example of this to illustrate 

 the method that may be very generally applied in most 

 of them. Let us suppose that a pipe of a certain dia- 

 meter branches out from another of a larger diameter at 

 a certain angle, as indicated on the longitudinal section 

 in Fitf. 31 ; and that we have to draw two elevations 

 and a transverse section of this, so as to show the form 

 of the joint, or intersection of the branch with the main- 

 pipe. 



In the first place, we should draw the centre lines or 

 axes of the cylindrical pipes, and then easily mark off 

 the general outlines and points of intersection in the 



Fig. 1. 



Hitherto we have only treated of the mode of drawing 

 outlines; we may now proceed to discuss tlio various 

 modes of rendering mechanical drawings distinct and 

 more easily intelligible. The.se may be said to consist 

 chiefly in hatching or section-lining, colouring, shading, 

 and shadowing. 



HATCHING. Hatching is merely drawing parallel 

 lines across the parts of any section, and are supposed 

 to be cut through the solid material. Fig. 32 is the 

 mere outline of the section of a hollow hexagonal prism ; 

 the lines convey no particular meaning, and they would 

 apply as well to the end elevation of a prism with another 

 projection from it ; but if the section through the solid 

 matter be hatched, as in Fig. 33, 

 Fig. 31. 



as if to indicate the appearance 



of the fibres of the cut material, 



Fig. 33. 



' 



two elevations. The points e, e, in the transverse sec- 

 tion, would be determined by setting off from /, on the 

 centre line, the inside radius of the small pipe, as de- 

 termined by the line f g in the longitudinal section. 

 These points would determine c 2 in the longitudinal sec- 

 tion, by tracing the vertical line upwards till it meets 

 the oblique centre Iine/c 2 in the longitudinal section. 

 Aud in a similar manner, by tracing up a line from 6 

 on the transverse section, we should find a point im- 

 mediately below 6 t on the side elevation, where the 

 outsides of the two pipes intersect on the oblique centre 

 line of the branch. There still remains for us to de- 

 termine the particular curves of the intersection lying 

 between the points so fixed ; and this we must do by 

 finding a number of other points in the different projec- 

 tions, which shall be the proper representations of points 

 in the circumferences of -the large and smaller pipes 

 where they intersect each other. Let us, for instance, 

 endeavour to determine some point in the curve of iii- 

 tersection represented on the side elevation ; and as this 

 must be a point on the outside of both pipes, we shall 

 draw their outside circumferences from the centres B 

 and C, connected by a line. Taking any point d in the 

 circumference of C, and tracing on to d l on the circum- 

 ference of B, and drawing the perpendiculars d e and 

 rf, e,, we draw on the side elevation H H parallel to the 

 axis of the small pipe at the distance C e from that axis, 

 and K. K parallel to the axis of the largo pipe, at the 

 distance B ,. The point d. J; where these lines H H and 

 K K intersect, is the projection of a point in the inter, 

 section of the circles on the side elevation. Tracing this 

 across by a horizontal line to the front elevation, cutting 

 iU centre line at e a , and setting off e, d s equal to d,, 

 we get d t a* the projection of the same point on the 

 Omit elevation. And thus, by projecting a number of 

 omU, we should be enabled to trace through them a 

 correct projection of the curves of intersection in their 

 different aspect*. 



" " 



it is understood that the drawing 

 represents a section. 

 Hatching is also useful in some cases for distinguishing 

 the different pieces of which the body drawn in section 

 consists. This is effected by hatching one of the pieces 

 by oblique lines lying in one direction, and an adjacent 

 piece by lines lying in another direction. Thus, Fig. 34 

 Flg M may represent a section of 



a vessel with a separate 

 cover fitted to it, and one 

 _ of the bolts and nuts used 

 | for holding the cover in its 

 F place. The hatching lines 

 of the vessel and its cover 

 are oblique in opposite 

 directions, and those of the 

 bolt and its nut are verti- 

 cal and horizontal respec- 

 tively, so that all the 

 separate pieces are dis- 

 tinguished. 



Hatching in colours, or coloured lines, is sometimes 

 useful to indicate the material as well as the separate 



Pieces. Thus, hatching parts of a section by black or 

 lue lines may indicate cast or wrought- tig, 35. 

 iron ; by rod or yellow, copper, brass, or 

 gun-metal. Sections of wood are gene- 

 rally hatched as in Fig. 35, by curved 

 lines, imitating, somewhat, the appearance 

 of the rings seen in timber when cut 

 across. Two separate pieces of timber seen 



Fig. 36. in cross section, side 



by side, may bo 



hatched by i 

 lying in "m 



us (Fig. 

 while timlicr in 

 ttulinal section, may be hatched by crooked discontinuous 

 linrs, lying generally length v. 



In working-drawings that is to say, projections 



