lo Rayleigh, TJic Mechanical Pri7iciples of Flight. 



According to a theoretical formula developed on the 

 basis of Kirchhoffs analysis {Phil. Mag., loc. cit.), we should 

 have for the mean pressure, instead of (4), 



7rsin« 



p = ^ 9l"~ (S). 



■^ 4 + 7rSina' ^-^ 



This applies strictly to motion in two dimensions, or prac- 

 tically to the case of a very elongated blade, whose length 

 is perpendicular to V. 



At perpendicular incidence (0 = 90°) the difference 

 between (4) and (5) is not important ; but when o is small, 

 the value of/ in (5) may be enormously greater than the 

 corresponding value from (4). 



As regards numerical values, if we use C.G.S. measure, 

 so that V \s measured in centimetres per second, we have 

 in the case of air under standard conditions p = '00128, 

 and/, at perpendicular incidence, measured in dynes per 

 square centimetre, is according to (4), 



p= '00064 ^'^ " (6). 



This does not differ greatly from the data given in 

 engineering tables. To compare with Langley's more 

 recent experiments, we may express V in metres per 

 second and/ in grams weight per square centimetre. Thus 



/- 00657"- (7): 



while the mean of Langley's numbers gives 



/ = -ooS7F'- (8), 



about 30 per cent, greater. The difference is accounted 

 for, at any rate partly, by the suction which experiment 

 shews to exist at the back of the plate. 



As regards the law of obliquity, the early experiments 

 of Vince (1798) sufficed to shew that the effect was more 

 nearly as sin a than as sin'^a. In recent times this subject has 

 been very thoroughly investigated by Langley, who has 



