L98 SMITHSONIAK CONTBIBUTIONS TO KNOWLEDGE VOL. 27 



greater the length, until with a piece 11 feet long, the full length of a cross- 

 rib, the twisting was so serious as to make the rib practically useless. It was 

 at first though! that this twisting might be overcome by making the webs slightly 

 wider, and it would to a certain extent, but in looking ahead and planning bow 

 the cross-ribs were to he fastened to the main ribs, the I-beam section was seen 

 to present so many difficulties that it was thought hardly worth while to spend 

 time on further experiments with it. This decision was made all the more im- 

 perative by foreseeing the difficulty of bending the I-beam section to the curve 

 which the cross-ribs were to have. In fact it had been found by experience 

 that while many different forms of ribs could be bent to the proper curve by 

 steaming and clamping them over a form and then drying them out while still 

 clamped to the form, yet the grain of the wood varied so in different ribs, that 

 of a dozen steamed and bent over the same form it was seldom that as many 

 as three would have approximately the same amount of curvature when re- 

 moved from the form after drying. If, however, the curve was formed in the 

 ribs by making them in two parts, which were glued together and clamped up 

 on the form while the glue dried, practically any number could be made which 

 would have tbe same curvature when thinned down to the proper thickness of 

 wall. 



It was recognized at all times that the gluing together of the ribs not only 

 entailed extra work, but introduced an element of uncertainty unless some 

 kind of a varnish for the ribs could be found which would prevent any possi- 

 bility of the glue becoming soft from moisture in the atmosphere or from the 

 wings actually coming down into the water when the aerodrome was tried in 

 flight. A search was therefore made for a varnish that was water-proof. A 

 large number of different varnishes were tried, and one was finally found which, 

 after repeated tests, seemed to be thoroughly good. Several test ribs were 

 given three coats of this varnish, and were then kept immersed in water for 24 

 hours without the glue showing any signs of softening. It was therefore de- 

 cided to follow the plan of gluing the ribs together and protecting them with 

 three coals of this varnish, which seemed to possess the remarkable properties 

 of being not only impervious to water, but also unaffected by the application 

 of concentrated ammonia or of gasoline, either of which produces immediate 

 softening when applied to ordinary varnishes. 



Following the indication of these tests thai the hollow, round section, 22 

 mm. outside diameter by 17mm. inside diameter, would probably give the best 

 cross-rib for the weighl thai it seemed possible to allow, a sel of cross-ribs of 

 tins Form was constructed and put in place in the large experimental wing, in 

 which the former solid ribs bad been tested. Tbe wing was inverted and fast- 

 ened into two posts at the angle it would have in flight, the guy-wires from the 



