condor is 14,336 times as heavy as the
humming-bird. What an amazing disproportion of weight! Yet by the same
mechanical use of its wings the condor can overcome the specific gravity
of its body with as much ease as the little humming-bird. But this is
not all. We are informed that this enormous bird possesses a power in
its wings, so far exceeding what is necessary for its own conveyance
through the air, that it can take up and fly away with a whole sheer in
its talons, with as much ease as an eagle would carry off, in the same
manner, a hare or a rabbit. This we may readily give credit to, from the
known fact of our little kestrel and the sparrow-hawk frequently flying
off with a partridge, which is nearly three times the weight of these
rapacious little birds.'

After a few more observations he arrives at the following conclusion:
'By attending to the progressive increase in the weight of birds, from
the delicate little humming-bird up to the huge condor, we clearly
discover that the addition of a few ounces, pounds, or stones, is no
obstacle to the art of flying; the specific weight of birds avails
nothing, for by their possessing wings large enough, and sufficient
power to work them, they can accomplish the means of flying equally well
upon all the various scales and dimensions which we see in nature. Such
being a fact, in the name of reason and philosophy why shall not man,
with a pair of artificial wings, large enough, and with sufficient power
to strike them upon the air, be able to produce the same effect?'

Walker asserted definitely and with good ground that muscular effort
applied without mechanism is insufficient for human flight, but he
states that if an aeronautical boat were constructed so that a man could
sit in it in the same manner as when rowing, such a man would be able to
bring into play his whole bodily strength for the purpose of flight,
and at the same time would be able to get an additional advantage by
exerting his strength upon a lever. At first he concluded there must
be expansion of wings large enough to resist in a sufficient degree
the specific gravity of whatever is attached to them, but in the second
edition of his work he altered this to 'expansion of flat passive
surfaces large enough to reduce the force of gravity so as to float
the machine upon the air with the man in it.' The second requisite is
strength enough to strike the wings with sufficient force to complete
the buoyancy and