AVIATION IN FRANCE JOURDAIN. 157 



plays havoc with them. The circular traction, even in a light-weight 

 propeller may be enough to shatter it or to tear out one of its blades. 

 On the other hand, this circular traction gives a great rigidity to the 

 materials used, inasmuch as the speed of rotation, being considerable, 

 permits the use of propellers of very thin wooden blades, or of sheets 

 of aluminum so thin that at rest they are actually supple. 



M. Chauviere has specialized in the study of this phase of the 

 question. He has built propellers of turned wood, that are quite 

 novel. 



This, however, is not the whole problem. At present we are using 

 propellors with short pitch and a high speed, which do not give 

 good results. Judging from our experience with steamships, this is 

 because the short pitched propeller, turning too rapidly, creates a 

 neutral space or vacuum in front of it and therefore does not take 

 full hold of the medium in which it turns. In water it turns in 

 its place without hardly advancing; this is what is called in French 

 the phenomenon of " cavitation." If the pitch be increased, there 

 is generated a reversing force which can not be neglected even with 

 the great inertia of aeroplanes. On the other hand, without altering 

 the pitch of the propeller the efficiency may be enhanced by increasing 

 its diameter, and the consequent volume of air upon which it acts. 

 But here we encounter still another difficulty that arises from the 

 necessary position of the driving apparatus in the flying machine, a 

 position determined by other mechanical considerations, and this diffi- 

 culty is embodied in the fact that a propeller's diameter must be so 

 limited that it will not touch the earth when the machine is on the 

 ground. 



M. Voisin, who is as well informed as anyone on this subject, has 

 mentioned having noticed in single long-pitched propellers, a partial 

 elimination of the reversing force by the reaction of the spiral of 

 air on the posterior compartment of the aeroplane; but nevertheless 

 there is a marked tendency among aviators, which will probably be 

 realized during the year 1909, to use two long-pitched propellers 

 turning slowly. 



To come into popular use, the aeroplane should satisfy three neces- 

 sary conditions. It should be easy to manage, it should not be too 

 expensive, and finally it should be of some actual service. 



We may already say that the machines are not extremely difficult 

 to manage, and that, therefore, is not a condition at which we should 

 stop. M. Delagrange is a sculptor; he had never had experience in 

 aviation and yet he quickly attained very satisfactory results. And, 

 if M. Voisin is to be believed, M. Moor-Brabazon made even more 

 remarkable a debut. 



Apprenticeship must certainly be longer in the Wright machine. 

 Mr. Wright has undertaken to teach pupils in three months. One of 



