150 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
to importance, the principal quality in an air-ship, without which it 
is but the plaything of the winds, and it is toward the improvement 
of this feature that all efforts should be directed. 
How may this individual velocity be obtained? In dirigibles all 
resistance to forward movement must be diminished as far as pos- 
sible; this is accomplished by appropriate design of form. This form 
must be permanently maintained, powerful motors must be pro- 
vided, driving good propellers. To sum up, every improvement 
which can be devised with regard to dirigible balloons should be 
directed principally if not solely toward the increase of their indi- 
vidual velocities. 
The same is true with regard to aviation apparatus, but in this 
direction the difficulty is much less, for because of their light design 
they offer much less resistance to forward motion than the dirigibles, 
condemned to drag along their enormous bags filled with hydrogen. 
For a given motive power therefore the former can attain much 
greater speeds than balloons, as experience has superabundantly 
demonstrated. 
However that may be, for the one or the other the individual 
velocity is a question of motive power, and since in an air-ship only 
a limited weight can be allowed for the motor, this must have as 
great a specific power as possible; in other words, the weight of the 
gas engine should be reduced as much as practicable. The question 
of individual velocity thus depends on the lightness of the motors. 
This motive power must, furthermore, be utilized to the best possible 
advantage, which can be done by proper propellers. ‘The resistance 
to forward movement must be diminished, and this can be accom- 
plished by careful design. The air-ship must also be stable in all 
directions—horizontally, longitudinally, or transversely—for yawing, 
pitching, and rolling, apart from the wearying effect on passengers 
and the dangers they may present, are formidable obstacles to the 
best speed. When a dirigible moves sidewise it presents an enormous 
surface to the air of a shape deplorable from the point of view of 
resistance, and the speed is diminished to an inconceivable degree. 
One can almost sum up in a word what can be said about indi- 
vidual velocity. It is this, that for an air-ship to possess this quality 
in the highest degree it must be endowed with all the others. 
There remains now the carrying capacity. Here the question 
appears in quite a different light, according to whether the aati ea 
is lighter or heavier than air. 
With a dirigible it is simply a question of the volume of the 
balloon. It must not be thought, however, that by increasing in- 
definitely the volume of the gas envelope that the carrying power 
of an air-ship can be increased without limit. To enlarge the volume 
means to increase the fabric surface and this will demand greater 
