152 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
and even more. We should, therefore, endeavor to increase the car- 
rying power per square meter of the sustaining planes. 
This carrying power may be increased partly by an increase in the 
sustaining quality of the bearing surface, and it is research in this di- 
rection that practically leads to the perfection of devices heavier than 
air. It is a question of form, dimensions, and orientation which 
must be taken up in detail. This problem constitutes in reality nine- 
tenths of the problem of aviation. 
In another way the load that can be carried per square meter of 
sustaining surface in a given apparatus, increases with the available 
motive power. The greater this power is in comparison with the 
weight of the machine the larger may be the load imposed on each 
square meter of sustaining surface. The increase is not proportional, 
but it is rather rapid, as may be shown by a few figures. If an aero- 
plane provided with a 25-horsepower motor can carry 10 kilograms 
per square meter, the same aeroplane with a motor of 50 horsepower 
ean carry 16 kilograms; with a 75-horsepower motor, 21; and with 
a 100-horsepower motor 25 kilograms per square meter. 
There is one very interesting point to note here, and that is, for a 
given aeroplane the capacity per square meter varies with the veloc- 
ity. Let us suppose that our aeroplane, with a 25-horsepower motor 
and carrying 10 kilograms per square meter, makes a speed of 60 
kilometers per hour. When it is provided with a motor of 50 horse- 
power, which will permit it, as we have just seen, to carry 16 kilo- 
grams per square meter instead of 10, its velocity will be increased. 
It will no longer be 60, but 76 kilometers per hour. In the same way, 
if it has a 75-horsepower motor due to which its carrying capacity 
increases to 21 kilograms, its velocity will at the same time reach 86 
kilometers. Finally, with the motor of 100 horsepower and a load 
of 25 kilograms per square meter it will have a velocity of 95 kilo- 
meters. 
The individual velocity and the carrying capacity therefore in- 
crease with the power of the motor; nothing of the kind occurs with 
dirigibles. 
However that may be, whether dirigibles or aviation apparatus are 
concerned, the carrying capacity is dependent on the lightness of the 
motors and the general perfection of the whole device; but these 
features have a much greater effect in the heavier than air system 
than in the lighter than air. In dirigibles there intervenes in this 
question a preponderating element, that of the volume of the gas bag 
whose influence dwarfs all others. This element does not exist in 
the aviation devices. 
We have still to examine stability in all its forms, but, as we have 
already seen, this property is indispensabie if we desire to attain an 
individual velocity of any magnitude whatever. There is, therefore, 
