HUXSAKER— EXGIXEERIXG ASPECTS. 251 



to know now is what kinds of gusts are to be expected. For 

 example, if sudden horizontal shifts in the direction of the wind 

 are the usual state of affairs, we should not put a great preponderance 

 of vertical fin surface on the tails of our aeroplanes. An excessive 

 " weather-cock " propensity will make a machine head into the rela- 

 tive wind and if the wind direction shifts constantly it will be diffi- 

 cult to maintain a straight course. The " weather-cock " stability 

 is of course provided to make steering easier. 



6. Also Professor \Mlson shows that a roller in the air is cer- 

 tain to bring disaster to an aeroplane. We have evidence of rotation 

 in the eddy formed in the lee of a hill or other obstruction, but there 

 is little information as to the extent and intensity of the disturbance. 

 \Miat aviators call " bad air " may be eddies in the wind. 



7. I would appreciate the opportunity to outline in a general way 

 some of the problems of lighter-than-air craft, airships and balloons, 

 in order to make the symposium more complete. 



8. In the design of airships we are confronted with indeterminate 

 structural features, mysteries of the upper air, atmospheric elec- 

 trical phenomena, and in addition to these difficulties we must work 

 with fabrics and membranes of unfamiliar and indefinite physical 

 properties. 



9. The theory of hydrogen-filled balloons was developed in a 

 very elaborate and complete form by the pioneers of the French 

 Army Engineering Corps. Their theoretical considerations are of 

 the greatest practical utility but depend upon an assumed stable con- 

 dition of the atmosphere. Unfortunately a balloon and to a less 

 degree a dirigible or airship is extremely sensitive to changes of 

 equilibrium. For example, a balloon floating at its zone of equi- 

 librium has exactly the weight of the air displaced. A wet cloud 

 may condense a little water on its surface, the balloon will sink into 

 regions of more dense air which will compress its volume and cause 

 continued descent until ballast is released or the ground reached. 



10. An airship is also handicapped by changes of weight in the 

 air due to picking up loads of condensed water, snow, or sleet. The 

 balloon fabric should be proofed in some manner to prevent such 

 accumulations. 



11. At the same time, though weight may not change, tempera- 



