Fig. 300.

In brush-shading as with line-shading, the difficulties increase with an increase in the size of the piece, and the learner will find that after he has succeeded tolerably well in shading these small pulleys, it will be quite difficult, but excellent practice to shade the large pulley in Figure 302.

One of the principal considerations is to not let the color dry at the edges in one part while continuing the shading in another part of the same surface, hence it is best to begin at the edge or outline of the drawing and carry the work forward as quickly as possible, occasionally slightly wetting with water edges that require to be left while the shading is proceeding in another direction.

Fig. 301.

When it is required to show by the shading that the surfaces are highly polished, the lighter parts of the shading are made to contain what may be termed splashes of lighter and darker shadow, as in Figure 303, which represents an oil cup, having a brass casing enclosing a glass cylinder, which appears through the openings in the brass shell.

Figure 304 represents an iron planing machine whose line-shading is so evenly effected that it affords an excellent example of shading. Its parts are similar to those shown in the iron planer in Figure 297, save that it carries two sliding heads, so as to enable the use, simultaneously, of two cutting tools.

Fig. 302.

A superior example in shading is shown in Figures 305 and 306, which represent a plan and a sectional view of the steam-cylinder of a Blake's patent direct-acting steam-pump. The construction of the parts is as follows: A is the steam-piston, H 1 and H are the cylinder steam-passages; M is the cylinder exhaust port.

Fig. 303.

Fig. 304. (Page 282.)

Fig. 305.

The main valve, whose movement alternately opens the ports for the admission of steam to, and the escape of steam from, the main cylinder, is divided into two parts, one of which, C, slides upon a seat on the main cylinder, and at the same time affords a seat for the other part, D, which slides upon the upper face of C. As shown in the engravings, D is at the left-hand end of its stroke, and C at the opposite or right-hand end of its stroke. Steam from the steam-chest, J, is therefore entering the right-hand end of the main cylinder through the ports E and H, and the exhaust is escaping through the ports H 1, E 1, K and M, which causes the main piston A to move from right to left. When this piston has nearly reached the left-hand end of its cylinder, the tappet arm, T, attached to the piston-rod, comes in contact with, and moves the valve rod collar O 1 and valve rod P, and thus causes C, together with the supplemental valves R and S S 1, which form, with C, one casting, to be moved from right to left. This movement causes steam to be admitted to the left-hand end of the supplemental cylinder, whereby its piston B will be forced towards the right, carrying D to the opposite or right-hand end of its stroke; for the movement of S closes N (the steam-port leading to the right-hand end), and the movement of S 1 opens N 1 (the steam-port leading to the opposite or left-hand end), at the same time the movement of V opens the right-hand end of this cylinder to the exhaust, through the exhaust ports X and Z. The parts C and D now have positions opposite to those shown in the engravings, and steam is therefore entering the main cylinder through the ports E 1 and H 1, and escaping through the ports H, E, K and M, which causes the main piston A to move in the opposite direction, or from left to right, and operations similar to those already described will follow, when the piston approaches the right-hand end of its cylinder. By this simple arrangement the pump is rendered positive in its action; that is, it will instantly start and continue working the moment steam is admitted to the steam-chest, while at the same time the piston is enabled to move as slowly as the nature of the duty may require. It will be noted that in Figure 305, the ports of C are shown through D, whose location is marked by dark shading. This obviously is not correct, because D being above C should be shaded lighter than C, and again the ports E 1 and K could not show dark through the port D. They might, of course, be shown by dotted outlines, but they would not appear to such advantage, and on this account it is permissible where artistic effect is sought, the object being to subserve the shading to making the mechanism and its operation clearly and readily understood.

Fig. 306.

Figure 307 affords another excellent example for shading. It consists of an independent condenser, whose steam-cylinder and valve mechanism is the same as that described with reference to Figures 305 and 306.

Fig. 307. (Page 288.)

Fig. 308. (Page 289.)

Fig. 309. (Page 289.)

Fig. 310 - Section of Cylinder and Steam Chest. (Page 289.)