SUPERIORITY IN AN AIR-SHIP—RENARD. 145 
traveler’s comfort is easily increased. These are questions to be 
referred to the skill of an upholsterer and not to an engineer. 
There is, however, one property that is highly important for safety 
and comfort in a voyage—the stability of the vehicle. This stability 
is obtained by mechanism of a technical nature; it is often very difli- 
cult to obtain and therefore should be considered in connection with 
the more exact qualities first discussed. In a given vehicle, stability 
can be interpreted in several ways. The center of gravity of the 
apparatus can describe a very regular trajectory, but the vehicle may 
nevertheless be exceedingly unstable; it may go through oscillatory 
movements which are highly uncomfortable and occasionally danger- 
ous. These movements have been given different names according to 
the direction they follow. When they are in a horizontal plane they 
are said to be zigzag movements or yawing. If it is a question of 
vertical movement, it may be of two sorts—in a longitudinal direction 
it is called pitching, and if in a tranverse direction it is rolling. 
Although displacements of this kind do not affect the trajectory of 
the center of gravity, and consequently can not prevent the vehicle 
from following its course, they are none the less disagreeable, espe- 
cially if several of them are combined. Stability of direction, longi- 
tudinal stability, and transverse stability, which will enable us to 
avoid, respectively, yawing, pitching, and rolling, are therefore 
qualities highly desirable. 
There is a fourth sort of stability that is a special quality of air- 
ships. This is stability of altitude. Land vehicles are forced to 
keep to the level of the ground on which they rest. Aquatic carriers 
float on the surface of the water; air-ships, on the contrary, and with 
them must be classed submarines, are submerged in a fluid and can 
ascend and descend through the gaseous or liquid mass. When the 
air-ship remains at the altitude chosen by the pilot, or when it mounts 
or descends at his will, it is said to have stability of altitude. It does 
not have this quality when its vertical movements are involuntary and 
beyond the control of the aeronaut. 
i. 
We have thus completed the enumeration of the qualities which an 
air-ship may possess. The question is now to choose from among 
them those most important in determining the value of the conveyance. 
But before making this choice it is indispensable to know from what 
point of view it is to be made. One may inquire as to which of these 
qualities is the most difficult to obtain. If the technical standing of 
engineers were to be determined that would be the course to pursue, 
and we should proclaim the superiority of the constructor who had 
endowed his machine with the qualities which are the hardest to 
