An impression of one of the workrooms in a modern factory may be gained from a study of the photograph on Plate XIII. This was obtained in the Grahame-White factory at Hendon. In the foreground of the picture, resting upon trestles, is one of the sustaining planes of a machine. It is in its rough stage, prior to being covered with fabric, and the method of construction may be seen. The system is to use two or more main-spars, which run the whole length of the wing; and above and below these pass the thin wooden strips which form a series of ribs. Fig. 90, representing a monoplane wing in skeleton, should make clear this method of building. Although such a wing may appear frail, it has in reality a surprising strength. English ash and silver spruce, in carefully selected lengths, are the woods used chiefly in aeroplane construction, and calculations have shown them able to bear a greater strain, weight for weight, than would a framework made of mild steel. In the future, of course, when aircraft are built far larger, wood will cease to be employed--and partly for the reason that, when wing constructions of great sizes are needed, it will be impossible to obtain timber that can be cut into spars of a sufficient length and soundness. Then, probably, will come the day of high-tension steel, and we shall have a liner of the air built almost after the fashion of a leviathan of the sea; but for the making of a small machine, at any time, there should be an advantage in the use of wood. [Illustration: FIG. 90.--Construction of a Monoplane wing.] Returning to an inspection of Plate XIII, one may observe, immediately behind the men who are working upon the wing-frame, two completed floats for a hydro-aeroplane; while behind these there is a workbench; and in the far distance, the hull and the centre wing-sections of a craft under construction. To the right of the centre work-table may be seen a man seated at a sewing machine; he is busy with the lengths of fabric which will cover the wooden framework of the wings. Upon the extreme left of the photograph, partly completed and in the form of a girder construction of wood, is the body of an aeroplane. Put together scientifically and from sections of wood specially tested, a remarkable strength may be obtained by such a method of building. Fig. 91 shows how a girder aircraft body, supported by trestles only at its ends, may support from its centre, without yielding, a tray containing a number of heavy weights. [Illustration: FIG. 91.--Testing the girder-built body of an aircraft.]