As before stated, great care should be taken in designing and making the framework of flying machines, and no stone should be left unturned in order to arrive at the greatest degree of lightness without diminishing the strength too much; then, again, elasticity should be considered. If we use a thin tube all the material is at the surface, far from the neutral centre, and great stiffness is obtained, but such a tube will not stand so much deflection as a piece of wood; then, again, wood is cheaper than steel, and in case of an accident, repairs are very quickly and easily made. Wood, however, cannot be obtained in long lengths absolutely free from blemishes. It therefore becomes necessary to find some way of making these long members of flying machines of such wood as may be found suitable in the following table. +---------------------+-------------+-------------+----------+ | | Strength | Weight of a | | | | per Sq. In. | Cube Foot | Relative | | | in Lbs. | in Lbs. | Value. | +---------------------+-------------+-------------+----------+ | Alder, | ... | 50 | ... | | Apple, | ... | 49·562 | ... | | Ash, English, | 16,000 | 52·812 | 302·9 | | Ash, White, | 14,000 | 43·125 | 324·6 | | Bamboo, | 6,300 | 25 | 252 | | Beech, English, | 11,500 | 53·25 | 215·9 | | Birch, | 15,000 | 45 | 333·3 | | Box, African, | 23,000 | ... | ... | | „ France, | ... | 83 | ... | | Cedar, American, | 11,600 | 35·062 | 330·8 | | Deal, Christiania, | 12,400 | ... | ... | | Ebony, | 27,000 | 83·187 | 324·6 | | Elm, | 6,000 | 35·625 | 168·4 | | „ Rock, | 13,000 | 50 | 260 | | Fir, Norway Spruce, | ... | 32 | ... | | „ Dantzic, | ... | 36·375 | ... | | Hackmatack, | 12,000 | 37 | 324·3 | | Hickory, | 11,000 | 49·5 | 222·2 | | Ironwood, | ... | 61·875 | ... | | Juniper, | ... | 36·375 | ... | | Lance, | 23,000 | 45 | 511·1 | | Lignum-Vitæ, | 11,800 | 83·312 | 141·6 | | Lime, | ... | 50·25 | ... | | Locust, | 20,500 | 45·5 | 450·5 | | Mahogany, Honduras, | 21,000 | 35 | 600 | | „ Spanish, | 12,000 | 53·25 | 225·3 | | Maple, | ... | 46·875 | ... | | Oak, African, | 9,500 | 51·437 | 184·7 |