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 effect, when money and brains are
diverted from the driven plane to a closer imitation of natural
flight.

Reverting from non-success to success, from consideration of the
two methods mentioned above to the direction in which practical