SUPERIORITY IN AN AIR-SHIP—RENARD. 149 
a given weight, but with dirigibles the principal method of reaching 
higher altitudes is by increasing the dimension of the gas envelope. 
An aerial voyage can be prolonged as long as supplies remain avail- 
able, whether the air-ship be lighter or heavier than air. The most 
important of these supplies is fuel for the motor and the accessory 
lubricating oils, the weight of which is comparatively small. In 
dirigibles there must also be a supply of ballast proportionate to the 
length of the voyage. The quality of duration is therefore a question 
of transporting capacity, and the methods of obtaining it are the 
same. 
The distance that can be covered is evidently proportional to the 
duration of the trip, and is also proportional to the absolute velocity 
of the air-ship. We have just considered the duration; as for the 
absolute speed, it is a quality that must be considered by itself. We 
have, therefore, as regards distance only one thing to keep in mind, 
and that is, it is obtained by combining the means used to attain 
duration and velocity. 
As already stated, absolute velocity is a resultant of two velocities, 
that of the wind and that of the air-ship; with the wind we can do 
nothing, but the individual velocity is another matter. It may be 
remarked that if the individual velocity should be less than that of 
the wind, the machine would not advance but would recede more or 
less from its point of departure. Such a machine, however, would not 
be dirigible and would not deserve the name of “ air-ship.” We mean 
to consider here only devices really dirigible, that is, those whose 
velocity is greater than that of the prevailing wind. In this case, 
whether flying against the air or with it, the absolute velocity will 
increase with the individual velocity. Let us suppose that the wind 
blows 50 kilometers an hour. The air-ship with an individual velocity 
of 60 kilometers will make 10 kilometers an hour against the current 
and 110 with it. If it has an individual velocity of 70 kilometers, it 
can travel 20 kilometers an hour against the wind and 120 with it. 
In either case it is evident that the absolute velocity increases with 
the individual velocity. One can even demonstrate mathematically 
that when an air-ship describes a closed circuit corresponding ap- 
proximately to the form of a circle or a regular polygon, whatever 
may be the velocity and direction of the wind the one that possesses 
the greatest individual velocity will have the greatest average abso- 
lute speed around the whole course. 
As we can not affect the velocity of the wind, to seek to increase 
the absolute velocity is in fact to seek the greatest individual velocity. 
We have just seen how desirable this quality of speed is in itself. 
Without it dirigibility is impossible; and the greater it is the more 
frequent are the occasions when we can travel in all directions and the 
greater will be the distances covered. Speed is, therefore, in respect 
