It is evident, too, that it might be desirable to show the arrangement of the seeds in the apple as they would appear if it was cut through in the other direction, say on the line _a b_, fig. _A_, and as is shown at _D_. There are therefore two kinds of sections; one _C_, in which the object is supposed to be cut through vertically, and therefore called a _vertical section_, the other when the object is supposed to be cut through horizontally, and therefore called a _horizontal section_, as shown at _D_. It is also evident that in looking at a locomotive or any other object, the appearance of the engine depends upon our position in relation to it. Thus, if we stand on the side of it, we see that part of the engine, and a drawing which represents the side, is therefore called a _side view_ or _side elevation_. A drawing which represents a locomotive or other object as it would appear to us if we stood in front of it, is called a _front view_ or _front elevation_, and a representation of the back part of any object as it would appear to us if we stood behind it is called a _back view_ or _back elevation_. Plate I is a side view, Plate II a section, Plate III a top view or plan;[1] the vignette in the title page is a front view and fig. 71 a back view of a locomotive. If the drawing is made as the object would appear if it was turned upside down, and we were looking at it from above, then it is called an _inverted plan_. [1] The boiler of the locomotive is supposed to be removed in Plate III. It is obvious, too, that it is possible to make a great many different sectional views of nearly any object, especially of a machine. Thus, we could suppose a locomotive cut through vertically and lengthwise, as is shown in Plate II. Such a representation is called a _longitudinal_ section. A locomotive could also be cut through crosswise, as shown in fig. 40, which is called a _transverse section_. It is of course possible to represent a transverse section of a machine like a locomotive at a great many different points; for example, it could be shown as though it was cut through the smoke-stack as in fig. 40, or through the boiler farther back, as the latter is shown in fig. 42. Usually when a section is shown through a cylindrical object like a smoke-stack or boiler, it is shown through its centre. If, however, this is not apparent from the drawing or engraving, it should be stated at what point it is supposed to be taken, thus the section _D_ of the apple is on the line _a b_ of fig. _A_, and the section _C_ is on the line _c d_. In drawing sections, the parts which are supposed to be cut in two are usually shaded with parallel diagonal lines drawn at equal distances apart, as shown in the sections of the apple at _C_ and _D_. Sections are also sometimes represented with solid black surfaces, as in Plates II and III, and in the engraving of a pump in fig. 66.