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THE POPULAR SCIENCE MONTHLY. 



the motor. It is not till that is got that calculation should come 

 in, when it can find a sure starting-point, based on well-conducted 

 experiments and precise ideas, and its results will be susceptible 

 of an immediate verification. We have constantly employed this 

 prudent, jDositive method, and it only can give satisfactory results. 

 This motor, which is to fulfill at the same time the two conditions 

 so hard to reconcile, of great power and extreme levity, we shall 

 now try to describe. 



Fig. 1. — BrRD-LiKE Generatok-Motor and Propeller. 



The fact indisputably results from observations, from the posi- 

 tive experiments of M. Marey, from the studies of M. Espitalier, 

 and from our personal labors, that birds expend on an average a 

 motor exertion of 75 kilogrammes per unity of weight — a unity 

 comprehended between 3'6 and 12*5 kilogrammes — in rising verti- 

 cally one metre per second. Observe that we are talking of gross 

 work, not of useful work effected directly upon the air. Thus 

 Goupil, a respected authority, has found that the work of a horse- 

 power in the pigeon is given for a weight of 12'5 kilogrammes. 

 That is the manifest work, but not the work really developed by 

 the animal ; the wing, like the screw, in fact, makes only a weak 

 return. 



We select, then, the minimum unity of weight 3'5 kilogrammes 

 per horse-power which results from the experiment with our elec- 

 trical helicopter, because we know in advance that we can not 

 obtain the full return for the expenditure ; and in this weight we 

 must include that of the generator of energy, or of the propeller, 

 and all the accessories. 



It is impossible, in this necessarily brief study, to give the 



