on 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 with Castel was Professor Forlanini, whose design was for
a machine very similar to de la Landelle's, with two superposed screws.
This machine ranks as the second on the helicopter principle to achieve
flight; it remained in the air for no less than the third of a minute in
one of its trials.

Later experimenters in this direction were Kress, a German; Professor
Wellner, an Austrian; and W. R. Kimball, an American. Kress, like most
Germans, set to the development of an idea which others had originated;
he followed de la Landelle and Forlanini by fitting two superposed
propellers revolving in opposite directions, and with this machine he
achieved good results as regards horse-power to weight; Kimball, it
appears, did not get beyond the rubber-driven model stage, and any
success he may have achieved was modified by the theory enunciated by
Berriman and quoted above.

Comparing these two schools of thought, the helicopter and bird-flight
schools, it appears that the latter has the greater chance of eventual
success--that is, if either should ever come into competition with the
aeroplane as effective means of flight. So far, the aeroplane holds
the field, but the whole science of flight is so new and so full of
unexpected developments that this is no reason for assuming that other
means may not give equal