154 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
absolute velocity and the duration of the voyage. These two factors 
may play a varying role in the final result. 
The factor of duration is certainly less important than the velocity. 
To obtain duration the machine need not even be dirigible; a simple 
free balloon can possess this quality, while up to the present time it 
is the spherical balloons which have made the longest uninterrupted 
voyages, so that while recognizing the valuable index which the 
distance traveled affords in the estimation of the merit of an air- 
ship, still, of the two elements which go to make it up, we must 
attach more importance to the absolute velocity than to the dura- 
tion of flight. 
It should be recalled, however, that these two qualities are not 
fundamental. The absolute velocity itself depends on the wind and 
the individual velocity, and from our point of view it is only im- 
portant if it is attained by the caprice of the wind but in the direc- 
tion desired by the pilot. To accomplish this, there must be indi- 
vidual velocity, a fundamental property. 
The duration of flight is itself dependent on the carrying capacity. 
We must, therefore, conclude that of these two fundamental prop- 
erties it is the individual velocity that stands first and the capacity 
of transport takes second place. 
As stated in the beginning of this discussion, I have arrived at 
these conclusions simply from utilitarian considerations. If we ex- 
amine the question from the point of view of the difficulties to be 
overcome, what rank shall we assign to these two essential qualities 
of an air-ship? For an aeroplane the question is very simple; the 
difficulties are the same in acquiring one as in acquiring the other. 
With an increase in the individual velocity, the possible load per 
square meter of sustaining surface is increased. Consequently, in 
making an advance in one a gain is made in the other. The question 
can be summed up by saying that an aeroplane should be as perfect 
as possible; that is, it should be stable, have carrying surfaces en- 
dowed with the best sustaining qualities, a good propeller, and a 
powerful and light motor. If it possesses such perfection it can be 
used in any way desired; it can travel swiftly and yet carry a con- 
siderable weight that may be utilized either as useful load or to in- 
crease the duration of the flight. If its load is lightened its speed 
will be diminished, but its abundant motive power will enable it to 
ascend. To conclude, with a perfect aeroplane the aviator may ob- 
tain whichever quality he desires or combine them in whatever pro- 
portion he deems convenient. 
In the case of aeroplanes, therefore, we may say that the question 
of difficulties to be overcome is negligible, and that utilitarian con- 
siderations alone determine their value. In these machines it is the 
individual velocity, as it is in all other types, which is the most im- 
