In fact, we have a
machine on this simple principle in the familiar toy which, set
rapidly whirling, rises in the air. Why more attempts have not
been made to apply this system, with two sets of sails whirling in
opposite directions, I do not know. Were there any possibility of
making a flying-machine, it would seem that we should look in this
direction.
The difficulties which I have pointed out are only preliminary
ones, patent on the surface. A more fundamental one still, which
the writer feels may prove insurmountable, is based on a law of
nature which we are bound to accept. It is that when we increase
the size of any flying-machine without changing its model we
increase the weight in proportion to the cube of the linear
dimensions, while the effective supporting power of the air
increases only as the square of those dimensions. To illustrate
the principle let us make two flying-machines exactly alike, only
make one on double the scale of the other in all its dimensions.
We all know that the volume and therefore the weight of two
similar bodies are proportional to the cubes of their dimensions.
The cube of two is eight. Hence the large machine will have eight
times the weight of the other.
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