ARTIFICIAL FLIGHT. 54g 



proper angle and with the proper speed in order to make 

 it fly well, and, let it be remembered, every failure would 

 represent a fatal accident on a flying machine. Instead of 

 sailing uniformly, the glider will, as a rule, if fairly well 

 thrown, describe a series of undulations in which, as in waves 

 on water, the crests are narrower than the troughs : in short, 

 curves much resembling a trochoid. To a mathematician 

 it is a trifling transition from these crests to cusps, and 

 from cusps to loops, and both of these forms are represented 

 in the paths described by gliders, but the glider very often 

 comes to rest and then falls down backwards instead of 

 turning over and over. The general form of all the curves 

 can be easily explained by the variations of the pressure 

 with the relative velocity of the wind, and of the position of 

 the centre of pressure with the inclination ; and I would 

 suggest that a series of instantaneous photographs of the 

 path of a glider, from which the forces and couples acting 

 on it might be calculated, would afford an interesting means 

 of verifying the laws of aerial resistance, just as the path 

 of a planet affords proof of the law of gravitation. 



The question next arises : Do the undulations remain 

 constant in shape as the glider descends, or do the crests 

 become more pointed till they become loops, or does the 

 reverse take place? If the second happens, the glider is 

 longitudinally unstable and dangerous, while if the third 

 happens it is longitudinally stable and safe. If the first 

 happened the motion under proper initial circumstances 

 would be stable but periodic, and any puft of wind might 

 alter its character and cause the glider to upset, and more- 

 over, if started from rest (i.e., without initial velocity) the 

 glider would certainly upset when it again came to rest. 

 It is easy to build up an explanation of Lilienthal's fatal 

 accident from some such considerations as these ; seeing 

 that it is known that he actually was brought to rest by 

 the wind, and in trying to get way on by tilting the machine 

 forward, he fell right over and was killed. It is quite 

 possible that had Lilienthal's machine carried a motor 

 capable of restarting its forward motion at the critical 

 moment, he might have been saved. 



