548 SCIENCE PROGRESS. 



eleven stone man, he says that no soaring machine should 

 weigh more than sixty pounds, because it fags one out too 

 soon, and the strain brought on is too severe. Many people 

 seem to think that a machine can be fully tried in a few 

 hours ; as a matter of fact it is only after having handled 

 the apparatus for several long days for hours at a time that 

 one begins to understand it at all. 



The arrangement of two superposed planes used by 

 Lilienthal in his latest experiments has not been adopted 

 by Pilcher, although he took several sails from the top of 

 Lilienthal's hill on the machine shortly before the death of 

 its inventor ; the chief objection raised by Pilcher to the 

 double surface being its unsteadiness in windy weather 

 caused by the height of the upper surface — in short, its 

 too great statical stability. 



Longitudinal balance and stability. — Although it is 

 essential for a ship to be properly balanced, the shifting of 

 its centre of gravity slightly fore or aft would only cause 

 the vessel to sink at the bows or stern and, even if the 

 effect were considerable, it could easily be corrected before 

 it led to any disastrous consequences. In an aerial 

 machine, this longitudinal balance is much more difficult to 

 deal with, since shifting the centre of gravity forwards 

 causes the machine to dive downwards, and shifting it 

 backwards causes the planes to tilt upwards and the appar- 

 atus to lose headway, become uncontrollable, and perhaps 

 fall over backwards. 



These facts, besides many others, can be verified by 

 any one with the use of one or two gliders made of paper. 

 It is only necessary to fold a sheet of paper down the 

 middle so as to make it slightly V shaped and to load it 

 with a small piece of stick to see how difficult this balancing 

 is. Setting aside the fact that gliders have an obstinate 

 habit of turning on one side instead of flying straight, the 

 experimenter will probably lose patience before the glider 

 can be balanced so that it will sail down steadily at an acute 

 angle with the horizon. 



But even after the glider has been properly balanced a 

 certain amount of skill is necessary in projecting it at the 



