This was a time of proving and verifying all manner of mechanisms. Continually, the principal features of both the Wind Bird monoplane and the strange rotor-bladed gyroscope were given try-outs on dog planes, as the test planes were dubbed. And continually, "bugs" developed on both types of models. "Bugs" are what the aero-mechanics call those errors that always show up when air-minded inventors begin putting theory into practice. The air is still such an unknown quantity that when mankind tries to enlarge an air machine from a tiny model into a great forty-foot wing expanse, a hundred odds and ends of air troubles immediately develop. This has always been the case. Back in those early experimental days at Kitty Hawk and at Kill Devil Hill, the Wright brothers had to fight the error "bugs" that crept into the enlargement of their models. Those early Wright gliders that flew perfectly in models scaled to inches, proved whole-hearted flops when enlarged to man-sized models. By painful, slow degrees, the Wrights finally mastered the fact that in enlarging a heavier-than-air flying machine, one part can be enlarged to four times its size, while another part of the same model may have to be enlarged sixteen times in order to preserve the air balance.