164 The Problem of Flight : [February, 
clination with which the wing advances will have to be 
increased with the weight, and also the force in the same 
relative proportion. 
It is asserted by some naturalists, in explanation of 
effective wing adtion, that the feathers of a bird’s wing are 
made to underlap each other, so that in the downward 
stroke the pressure of the air closes them upwards against 
each other, and converts the whole series into one connected 
membrane, through which there is no escape ; whilst in the 
upward stroke the same pressure has precisely the reverse 
effedt. “ It opens the feathers,” says the Duke of Argyll, 
“ separates them from each other, and converts each pair of 
feathers into a self-adting valve through which the air rushes 
at every point.” The Duke, in his “ Reign of Law,” re- 
marks Mr. Brearey, so thoroughly recognises, in another 
place, the immense importance of the concave and convex 
surfaces in gripping the air in the one case and evading it 
in the other, that he can scarcely lay much stress upon the 
valvular system of feathers. Dr. Pettigrew, whose re- 
searches give weight to his statement, estimates this differ- 
ence as two to one. 
To show that in the wing propeller the convex and concave 
arrangement is most effective, Mr. Brearey stood upon a 
pivoted stool, and holding the artificial wing perfectly level, 
waved it up and down, by which adtion he was caused to 
revolve. 
He then proceeded to the pradtical application of the 
concave-convex theory by exhibiting a model after the con- 
strudtion of M. Penaud, of the French Society. 
Experimenting with various forms of wings, he has been 
enabled to imitate the leisurely flight of the crow and the 
swift flight of the swallow. Mr. Brearey also made some 
observations upon the vertical screw, again resorting to 
M. Penaud’s Helicoptere in illustration. 
Passing to the question of the use of balloons in Polar 
Exploration, the author remarked that the balloon has a 
sphere of its own quite independent of its shape, unap- 
proachable by any other invention, and the question for 
discussion is — Wa^ the late Polar Expedition such an oppor- 
tunity as afforded any reasonable chance for the useful 
employment of the balloon ? 
This question can never be fairly answered unless a bal- 
loon, with the necessary means for its inflation, form part of 
the vessels equipment. 
All the materials and apparatus being conveyed to the 
place of destination, there should be no difficulty in the 
