TJie Three Secretaries 225 



Charm te, of Chicago, responded to the writer, April 10, 1896, 

 as follows : 



"In my judgment the principal contributions thus far made 

 by Doctor Langley to the science of Aerodynamics consist in 

 his having given to physicists and searchers firm ground to 

 stand upon concerning the fundamental and much-disputed 

 question of air resistances and reaction. 



"When I was in Europe in 1889, I inquired into the state 

 of knowledge on this important question, and found utter dis- 

 agreement and confusion. There were numerous formulae, 

 promoted by various physicists, but these gave such discor- 

 dant results that arrangements were being proposed in France 

 to try an entire set of new experiments, with air currents to 

 be procured by an enormous fan-blower. A fair idea of the 

 state of knowledge can be had from Professor Marey's careful 

 work on "Le vol des oiseaux," published in 1890. Oblique 

 pressures were then still generally held to vary according to 

 the Newtonian law, or as the square of the sine of incidence, 

 although this gives but five to ten per cent, of the true reac- 

 tions at acute angles of incidence. 



" Doctor Langley has shown us, by experiment, the general 

 accuracy of which cannot be questioned, that the empirical 

 (based on experiments) formula of Duchemin is sufficiently 

 correct to calculate the radiations upon planes; so that the 

 French, who had ignored this formula since 1836, now claim 

 its inception and accept it (as they do some wines) retour 

 cTAmerique. Doctor Langley has also shown us that the va- 

 riation of the center of pressure on an inclined plane, observed 

 by Sir George Cayley and by Avanzani as well as by Kummer, 

 follows approximately the law formulated by Jossel, so that 

 now, for the first time, searchers are enabled to calculate the 

 sustaining power, the resistance, and the center of pressure 

 of a plane, with confidence that they are not far wrong ; and 

 this, together with the further law, formulated first by Doctor 

 Langley, that within certain limits 'the higher speeds are 

 more economical of power than the lower ones,' has made it 

 possible to assert that the problem of artificial flight is not in- 



