144 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
has not only done everything, as in calm air, but it has done more, 
for in addition to the absolute velocity it has had to furnish a surplus 
of individual velocity to counterbalance the hindering effect of the 
wind. 
In a word, in order to attain the same practical result as before, 
that is, an absolute velocity of 50 kilometers an hour, the motor 
should be capable of giving to the air-ship an individual velocity 
of 0, 20, 40, 50, or 80 kilometers an hour. 
We have considered here only the simplest case—when the wind 
blows in the direction of the place to be reached or in exactly the 
opposite direction. This is almost never the case in practice, so 
that it becomes necessary in each case to determine what the indi- 
vidual velocity must be to attain a certain absolute speed. The 
problem is now a little more complicated, but the conclusions are the 
same, and the individual velocity is necessarily sometimes less, some- 
times more, than the absolute velocity, and at times the two may 
even be equal. To sum up, all that may be said is that the wind 
ean be either a help or a hindrance to the progress of air-ships, and 
in exceptional cases neither obstructs nor is favorable to their evo- 
lutions. 
By those with a different point of view, it may finally be asked 
if there is not opportunity to measure the value of an air-ship by the 
amount of useful weight carried, in personnel or in material. The 
power of transporting is certainly one of the qualities sought for in 
certain vehicles. 
All the qualities which we have passed in review—altitude, dura- 
tion of voyage, distance covered, velocity, power of transportation— 
have the common characteristic that they may be measured exactly, 
their value can be expressed in precise figures, and thus they furnish 
a fixed mathematical standard of comparison between different types 
of air machines, for they are based on rigorous observations, and 
questions of sentiment have not intervened. For instance, if the 
altitude attained should be taken as the criterion of the value of a 
dirigible, the one that has ascended to a height of 1,500 meters is 
incontestibly superior to one that has only attained a height of 1.200 
meters. If it is a matter of distance covered, the one which in a 
single flight has traveled 800 kilometers is superior to one which 
has only covered 600. That much is perfectly clear. 
There are other qualities, however, less exact in their nature, which 
nevertheless are not negligible, such as security, comfort, and pleasure 
of voyages. 
T do not eare to enter into a detailed examination of these phases of 
the subject, partly because they can not be exactly valued, and further 
because they are readily attained by devices of secondary importance. 
Thus by the use of flexible cushions and backs with head rests the 
