502 FLIGHT AND FLYING MACHINES. 
horse-power, you will see that Mr. Maxim has certainly got sufficient 
motive power in proportion to weight ; and what is more conclusive is 
that he has actually been able to make his machine lift itself from the 
ground. ‘This proves that the purely mechanical difficulties of flight 
are not insuperable. Moreover, if a large machine, such as Maxim’s, 
will lift itself off the ground, what we have said shows that a smaller 
machine would have to carry less horse-power per lb. to accomplish a 
similar feat, and hence experiments made with a rather smaller and 
consequently more controllable machine might even answer better. 
Before Mr. Maxim constructed his machine he, and at the same 
time, Professor Langley, made a number of experiments on the action 
of aero-planes moving through the air. Professor Langley was ap- 
pointed to experiment on the subject by the Smithsonian Institution, 
of Washington, but Mr. Maxim made similar investigations quite 
independently at the same time, a whirling table being used in both 
cases. ‘The principle was this: a plane was made to revolve round a 
central axis and the force of pressure of the air on it carefully measured 
so as to discover the laws according to which the resistance of the air 
varied when the velocity or the inclination of the plane was varied. 
The first conclusion that these experiments brought out most 
prominently may be stated thus:— If a plane be held perfectly 
horizontally it falls to the ground less quickly if it has a horizontal 
motion imparted to it (as when made to travel round and round by 
means of this whirling table) than it would do if it were simply falling 
vertically. In other words, the horizontal velocity lengthens the time 
of fallmg. Let us enquire why this is; let us go back to our old 
problem of the recoil of a gun, and the result that we deduced there- 
from, namely, that in order for a body to support itself in the air it 
must impart a downward motion to as great a mass of air as possible. 
There are two ways of getting hold ofa great mass of air; one is by 
constructing an enormously large aero-plane or parachute; but there 
is another way as well. If we move a plane rapidly through the air in 
a horizontal direction it will come in contact with a great many 
different portions of air in succession, and in that way it will act ona 
far larger mass of air, simply on account of the rate at which it 
traverses the air. Another argument leads to very much the same 
result. This dropping plane, you see, imparts a little downward motion 
to the air as it drops, but as soon as it has got a little distance it comes 
in contact with fresh particles of air at rest; it 1s never supported for 
any length of time by particles to which it has already imparted a per- 
ceptible downward velocity ; it is, therefore, constantly striking air at 
rest, and accordingly is much more retarded than it would be by 
ordinarily making its way constantly through the same air that it has 
get in motion. The ordinary screw propeller of a ship affords another 
illustration of this point. Ifaship is at rest in port and the screw is 
set in motion, a man can nearly keep the ship from moving by holding 
it with his hand, and the screw only keeps churning the same portions 
of water, and so has little power; but as soon as the ship is under way 
the screw propeller comes in contact with fresh portions of water 
previously at rest, and so gets a much greater grip on the water than 
