FLIGHT AND FLYING MACHINES. 503 
when the ship was at rest inthe harbour. And just the same happens 
with this aero-plane. 
Another result which Professor Langley’s and Mr. Maxim’s experi- 
ments proved was this:—The horse-power required to sustain a given 
weight in horizontal flight by means of an aero-plane of given 
dimensions is less for high speeds than for low ones. In other words, 
the faster the aero-plane travels, the less energy is required to keep it 
up in the air for the same length of time. This fact doubtless accounts 
for the high speeds at which birds fly, and it depends upon the same 
principle as the first result, namely, that the more rapidly you drive 
the plane through the air, the more you bring it into contact with fresh 
particles of air. 
I will now just mention some further points in connection with these 
experiments. If we take an ideal plane surface going along against 
the wind, the total pressure is perpendicular to the surface, and by the 
elementary laws of mechanics we can resolve this force into two com- 
ponents, viz., an upward force, called the lift, which lifts or supports 
the weight of the plane, and a horizontal force called the drift, which 
represents the force with which the plane must be driven. By making 
the plane very nearly horizontal, we can decrease the drift to any 
extent ; but if the plane is perfectly horizontal and the wind will exert 
no pressure on it, we shall get no lift or drift at all; so that we can- 
not do away entirely with the drift; we must have some motive power 
to support a weight by means of our aero-plane. But, if instead of a 
plane surface, we experiment with a concave curved surface of the same 
area, itis found that we get greater lift and less drift, so that it ig 
possible to lift the same weight with less expenditure of horse-power 
than before, This result holds good provided that the curved surface 
is inclined at a moderately small angle to the horizon. When the 
curved surface is brought more nearly vertical, so that its inclination 
to the horizon is considerable, the reverse is the case; we get greater 
drift and less lift than with the plane surface. But this is exactly 
what we want; for in order to stop a flying machine we should have 
to bring the aero-plane nearly vertical as birds do with their wings. 
What we want is to check the horizontal speed as much as possible, 
accordingly we want the drift to be as great as possible, and here again 
the concave surface is the best. 
I have already mentioned that flapping wings are not so well adapted 
for any mechanical construction as the screw propeller. There is an 
essential difference between the mechanism of an animal and that of a 
machine ; in the former, continuous rotation is impossible, in the latter 
it is the simplest form of motion attainable. Mr. Maxim, in a recent 
lecture, remarked in replying to the question why he did not take the 
flight of birds as his model, that any man could easily, with a little 
ingenuity, construct a machine which would walk like a horse or would 
walk like a man at about four miles an hour; but when we have 
locomotives that will go 60 miles an hour, there would be no point in 
doing so. Similarly, when we have screw propellers, which are better 
adapted to a machine, and which, moreover, have proved so efficacious 
in the propulsion of ships, there is not much to be gained by trying 
