In this and many other cases a side view and an end view give all the outside dimensions needed but sometimes a top view must also be made, and this is shown in Fig. 18.
Fig. 19. A CROSS-SECTION OF A STEAM ENGINE
While all of these views show the outside of the steam chest and the cylinder they give no hint as to how the inside is made. Suppose you had invented the steam engine of course you would know how the inside should be made and so you make a cross-sectional drawing of the parts as shown at Fig. 19, and then the construction and even the operation of the engine looms up as though you had turned a searchlight on it.
Fig. 20. THE SIDE VALVE SHOWN IN DETAIL
That is all of it will be clear except perhaps the slide valve and this is where a detailed drawing comes in to show a small part, or a part that is hard to understand by looking at the side, end and top views. The slide valve, see Fig. 20, is drawn in detail and the picture is made large and bold. The slide valve is made of a cast piece of metal hollowed out. It and the completed steam chest and cylinder are both drawn in perspective, that is just as the eye would see them if they were actually made of metal. The latter is shown in Fig. 21.
Fig. 21. AN ISOMETRIC PERSPECTIVE DRAWING OF A STEAM ENGINE