142 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
to be considered. It is not enough merely to rise, however, but it is 
also necessary to stay there. The period during which the air-ship 
shall remain suspended in the air without touching the ground, there- 
fore, is also one of the elements of interest in the question. 
Another phase of the question is that any engine of locomotion must 
be able to cover distances; the distance which separates the point of 
departure from the finishing point is therefore one of the essential 
characteristics of a voyage. In fact one might be tempted to say that 
the best air-ship is the one that can travel the greatest distance in a 
single flight before touching the earth. 
Finally, it is not only necessary that a certain given distance shall 
be covered, but it must take the shortest possible time to accomplish 
it. In other words, speed is the most highly valued quality at the 
present day. In all types of locomotion, whether by bicycle, automo- 
bile, railroad trains, steamboat, or motor boat it seems that the prin- 
cipal aim is speed, always speed, and still more speed. This search 
for acceleration in means of transportation is one of the character- 
istics of our epoch; and it is not to be wondered at, for although all 
space is open to us, still our time is parsimoniously dealt out to us, 
and the best way we can use it is to carefully economize it by the use of 
the powerful mechanical means at our disposal. 
Aerial navigation does not escape from this general law of locomo- 
tion. Speed is therefore one of the important elements in the meas- 
urement of the value of an air-ship. But a distinction must here be 
made, for there are two kinds of speeds to be considered, termed 
absolute speed, and individual speed. The absolute or effective speed 
is the one commonly considered. It is the speed measured with re- 
gard to the ground over which the air-ship is passing. If a dirigible 
starts from Paris at 8 in the morning and at 11 o’clock is above Au- 
xerre, the distance between the two cities being 150 kilometers as the 
crow flies, we would say that its absolute velocity had been on the 
average 50 kilometers an hour. This absolute speed is the one of 
practical interest. It is the plain fact, all modifying circumstances 
being removed from the calculation. 
From the point of view of merit in a device, however, it is pre- 
cisely these modifying circumstances that should be considered. ‘The 
effective velocity results from the combination of two other velocities, 
namely, the individual velocity of the vehicle, which will be defined 
shortly, and the velocity of the wind. 
Everyone knows what the velocity of the wind means. As for the 
individual velocity of an air-ship, its definition is very simple; it is 
the velocity which the air-ship could attain if there were no wind, or, 
again, it is the velocity in calm air, or finally, its velocity in compari- 
son with the ambient air, considering this to be at rest. 
ct a 
