Walker's work as a whole is its
suggestiveness; he did not hesitate to state that the 'art' of flying is
as truly mechanical as that of rowing a boat, and he had some conception
of the necessary mechanism, together with an absolute conviction that he
knew all there was to be known. 'Encouraged by the public,' he says,
'I would not abandon my purpose of making still further exertions to
advance and complete an art, the discovery of the TRUE PRINCIPLES (the
italics are Walker's own) of which, I trust, I can with certainty affirm
to be my own.'

The pamphlet begins with Walker's admiration of the mechanism of flight
as displayed by birds. 'It is now almost twenty years,' he says, 'since
I was first led to think, by the study of birds and their means of
flying, that if an artificial machine were formed with wings in exact
imitation of the mechanism of one of those beautiful living machines,
and applied in the very same way upon the air, there could be no doubt
of its being made to fly, for it is an axiom in philosophy that the same
cause will ever produce the same effect.' With this he confesses his
inability to produce the said effect through lack of funds, though he
clothes this delicately in the phrase 'professional avocations and other
circumstances.' Owing to this inability he published his designs that
others might take advantage of them, prefacing his own researches with
a list of the very early pioneers, and giving special mention to
Friar Bacon, Bishop Wilkins, and the Portuguese friar, De Guzman. But,
although he seems to suggest that others should avail themselves of
his theoretical knowledge, there is a curious incompleteness about the
designs accompanying his work, and about the work itself, which seems
to suggest that he had more knowledge to impart than he chose to make
public--or else that he came very near to complete solution of the
problem of flight, and stayed on the threshold without knowing it.

After a dissertation upon the history and strength of the condor, and
on the differences between the weights of birds, he says: 'The following
observations upon the wonderful difference in the weight of some birds,
with their apparent means of supporting it in their flight, may tend
to remove some prejudices against my plan from the minds of some of
my readers. The weight of the humming-bird is one drachm, that of the
condor not less than four stone. Now, if we reduce four stone into
drachms we shall find the