The attraction of the helicopter lies, probably, in the ease
with which flight is demonstrated by means of models constructed
on this principle, but one truism with regard to the principles
of flight is that the problems change remarkably, and often
unexpectedly, with the size of the machine constructed for
experiment. Berriman, in a brief but very interesting manual
entitled Principles of Flight, assumed that 'there is a
significant dimension of which the effective area is an
expression of the second power, while the weight became an
expression of the third power. Then once again we have the
two-thirds power law militating against the successful
construction of large helicopters, on the ground that the
essential weight increases disproportionately fast to the
effective area. From a consideration of the structural features
of propellers it is evident that this particular relationship
does not apply in practice, but it seems reasonable that some
such governing factor should exist as an explanation of the
apparent failure of all full-sized machines that have been
constructed. Among models there is nothing more strikingly
successful than the toy helicopter, in which the essential
weight is so small compared with the effective area.'

De la Landelle's work, already mentioned, was carried on a few
years later by another Frenchman, Castel, who constructed a
machine with eight propellers arranged in two fours and driven
by a compressed air motor or engine. The model with which
Castel experimented had a total weight of only 49 lbs.; it rose
in the air and smashed itself by driving against a wall, and the
inventor does not seem to have proceeded further. Contemporary