at each stroke. So that the conditions of weight of air for
obtaining support equally well apply to weight of air and its
reaction in producing forward impulse.

'So necessary is the acquirement of this horizontal speed, even
in commencing flight, that most heavy birds, when possible, rise
against the wind, and even run at the top of their speed to make
their wings available, as in the example of the eagle, mentioned
at the commencement of this paper. It is stated that the Arabs,
on horseback, can approach near enough to spear these birds,
when on the plain, before they are able to rise; their habit is
to perch on an eminence, where possible.

'The tail of a bird is not necessary for flight. A pigeon can
fly perfectly with this appendage cut short off; it probably
performs an important function in steering, for it is to be
remarked, that most birds that have either to pursue or evade
pursuit are amply provided with this organ.

'The foregoing reasoning is based upon facts, which tend to show
that the flight of the largest and heaviest of all birds is
really performed with but a small amount of force, and that man
is endowed with sufficient muscular power to enable him also to
take individual and extended flights, and that success is
probably only involved in a question of suitable mechanical
adaptations. But if the wings are to be modelled in imitation
of natural examples, but very little consideration will serve to
demonstrate its utter impracticability when applied in these
forms.'