f thrust which might be obtained by
using very high speeds, and from this many were led to suppose that high
speed was essential to success in a motor-driven machine. But the
economy to which Professor Langley called attention was in foot-pounds
per mile of travel, not in foot-pounds per minute. It is the foot-pounds
per minute that fixes the size of the motor. The probability is that the
first flying machines will have a relatively low speed, perhaps not much
exceeding 20 miles per hour, but the problem of increasing the speed
will be much simpler in some respects than that of increasing the speed
of a steamboat; for, whereas in the latter case the size of the engine
must increase as the cube of the speed, in the flying machine, until
extremely high speeds are reached, the capacity of the motor increases
in less than simple ratio; and there is even a decrease in the fuel
consumption per mile of travel. In other words, to double the speed of a
steamship (and the same is true of the balloon type of airship) eight
times the engine and boiler capacity would be required, and four times
the fuel consumption per mile of travel; while a flying machine would
require engines of less than double the size, and there would be an
actual decrease in the fuel consumption per mile of travel. But looking
at the matter conversely, the great disadvantage of the flying machine
is apparent; for in the latter no flight at all is possible unless the
proportion of horse-power to flying capacity is very high; but on the
other hand a steamship is a mechanical success if its ratio of
horse-power to tonnage is insignificant. A flying machine that would fly
at a speed of 50 miles an hour with engines of 1,000 horse-power would
not be upheld by its wings at all at a speed of less than 25 miles an
hour, and nothing less than 500 horse-power could drive it at this
speed. But a boat which could make 40 miles per hour with engines of
1,000 horse-power would still move four miles an hour even if the
engines were reduced to one horse-power. The problems of land and water
travel were solved in the nineteenth century, because it was possible to
begin with small achievements and gradually work up to our present
success. The flying problem was left over to the twentieth century,
because in this case the art must be highly developed before any flight
of any considerable duration at all can be obtained.

[Illustration]

However, there is another way of flying