SUPERIORITY IN AN AIR-SHIP—-RENARD. 143 
Of these two elements, the combination of which determines the 
absolute velocity, one, the individual velocity, depends on the con- 
struction of the air-ship; and the efforts of all aeronautic engineers 
are directed toward giving this as great a value as possible; the other 
element, the velocity of the wind, is entirely beyond us and we must 
submit to it, whatever it is. But according to the direction and the 
velocity of the wind, it is necessary to have very different individual 
velocities to obtain a determined effective velocity. 
If, for example, on the day when our dirigible traveled from Paris 
to Auxerre in three hours, the wind had blown exactly in the desired 
direction with a velocity of 50 kilometers an hour, the wind alone 
would have been sufficient to accomplish the voyage in the time given 
without any intervention of the individual velocity. The aeronaut 
could have stopped his motor and thus would have made the journey 
at little cost. The effective speed would be the same as the velocity 
of the wind, the individual speed zero; the wind would have done all 
and the machine nothing. 
If the wind, however, although blowing in the proper direction 
from Paris to Auxerre, had had a velocity of only 30 kilometers an 
hour the aeronaut, if he were contented with allowing himself to be 
carried by the wind, would have taken five hours to make the journey 
instead of three. To attain the previous speed of 50 kilometers per 
hour he would have to add to the velocity of the wind the 20 kilome- 
ters lacking, and this difference would be nothing else but his indi- 
vidual speed. In such a case we should say that the velocity of the 
wind had been 30 kilometers an hour, the individual velocity 20, and 
the effective or absolute velocity 50 kilometers per hour. Instead of 
doing all the work as before, the wind had only done the greater part 
and the motor the rest. 
If the velocity of the wind had been but 10 kilometers, the motor 
this time would have had to add not 20 kilometers but 40. In this 
case the motor would have deserved the principal credit for the 
voyage, and the wind would have furnished only a slight supple- 
mentary velocity. 
Let us suppose now that the air is absolutely calm, that is, the 
velocity of the wind is zero. The motor alone can be counted on 
here, and it is due to it that the speed of 50 kilometers an hour is 
attained. The effective velocity will be equal to the individual 
velocity, and the motor will have done all and the wind nothing. 
Finally, if the wind, with a velocity of 30 kilometers an hour, 
is blowing not in such a direction as to be astern from Paris to 
Auxerre, but in the opposite direction, the motor will be required 
to furnish an individual speed of 80 kilometers an hour. The first 30 
are used up merely in compensating for the unfavorable effects of 
the wind, the other 50 alone being effective. This time the motor 
