The relative value of different kinds of wood is shown in this table, and it will be observed that some are much more suitable for the purpose than others. The true value of a wood to be used in flying machines is only ascertained by considering its strength in comparison with its own weight--that is, the wood which is strongest in proportion to its weight is the best. It will be seen that Honduras mahogany stands at the head of the list, but American white pine is very good for certain purposes, as it is light, strong, easily obtained, and takes the glue very well indeed. In Fig. 45, I have shown a good system of producing the long members necessary in flying machines. I will admit that it costs something to fit up and produce the kind of joints which I have shown, but when the members are once made, they are exceedingly strong and stiff. Fig. 46 shows sections of the struts, and these may be made of either straight-grained Honduras mahogany or of lance wood; either answers the purpose very well, because being very strong and straight-grained, permits the struts to be made of such a shape and size as to offer very little resistance in cutting their way through the air. The framework of the aeroplane unless carefully designed will offer great resistance to being driven through the air. Suppose that the bottom member of the truss (Fig. 47) is straight, and the top one curved in the direction shown; no matter how taut the cloth may be drawn, the pressure of the air will cause it to bag upwards between the different trusses, so as to present very nearly the correct curve which is necessary to produce the maximum lifting effect, and without offering too much resistance to the air; however, one must not forget for a single moment that the air flows over both sides of the aeroplane. When the aeroplane is made very thick in the middle and sharp at the edges (Fig. 48), with the bottom side dead level, it produces a decided lifting effect no matter which way it is being propelled through the air. This is not because the bottom side produces any lifting effect of itself, but because the air running over the top follows the surface. The aeroplane encounters air which is not moving at all. The air is first moved upwards slightly, but it also has to run down the incline to the rear edge of the aeroplane, so that, when it is discharged, it has a decided downward trend; therefore, the air passing over the top side instead of under the bottom side, produces the lifting effect, showing that the top side of an aeroplane as well as the lower side should be considered. The top side should, therefore, be free from all obstructions. [Illustration: Fig. 45.--System of splicing and building up wooden members. When they have to be curved and to keep their shape, they should be bent at the curve at the time of being glued together, and joined in the middle as at _d_.] [Illustration: Fig. 46.--Cross-section of struts.]