AEROPLANES.                              157
in the apparatus when exposed to a wind of about 13 miles
per hour, at which the .air pressure would be presumably
some 0.85 lbs. per square toot. The angle of incidence of
the wings was practically 10°, and we may, without seri-
ous error, assume the resistance of the body to have been
one-tenth of that due to «its mid-section, while that of the
edges of the wings (presumably 0.20 ft. in average thick-
ness) would be about one-third of their plane cross-sec-
tion. As the sustaining surface was 290 sq. ft., we then
have, using the table of "lilt" and " dnit" heretofore
given, the following estimate :
RESISTANCE OF  THE GOUPIL AEROPLANE.
Drift io°......290   x 0.85X00585      = 14.42 lbs.
Body........... 26.9 X 0.85 -f- 10           =   2.28  "
Edge of wings..   19.7X0.2   X 0.85 -^ 3 =    1.11   "
Total           17 81   "
which agrees closely with the amount said to have been
ascertained by experiment ; but when we come to calcu-
late the lifting force we have :
Lift 10° — 290 X 0.85 X o 332 = 82 lbs.,
while the apparatus is said to have actually lifted 440 lbs.,
or more than five times as much !
Of course various allowances must be made in consider-
ing the results of an experiment carried on in a variable
wind, and where so little motion of the apparatus (2 ft.)
could be allowed. The thrust may have been measured
while the breeze was steady, and the uplift to have oc
curred during a wind gust, deflected possibly by surround-
ing objects so as to produce a greater angle than 10° with
the wings ; still, in any case, the result of this experiment
and also of other experiments by M. Phillips, which are
to be described hereafter, leads to the inference that much
greater supporting power is to be obtained from concavo-
convex surtaces than from the flat planes which hitherto
have been chiefly proposed for aeroplanes.
This increase in supporting power might indeed have
been expected from the theoretical consideration : that the
concave lower surface would produce a higher co-efficient
of pressure, while the convex upper surface would deflect
the current of air impinging at an acute angle thereon)
and thus produce a partial rarefaction ; and also from the
much stronger practical consideration that this is the way
the wings of birds are shaped ; and yet very few experi-
ments and proposals seem to have been made with bird-
like aeroplanes.