perpendicular impulse of a plane is found to be in supporting a weight,
when there is no horizontal motion at the time. There is no material
weight of air to be acted upon, and it yields to the slightest force,
however great the velocity of impulse may be. On the other hand, suppose
that a large bird, in full flight, can make forty miles per hour, or
3,520 feet per minute, and performs one stroke per second. Now, during
every fractional portion of that stroke, the wing is acting upon and
obtaining an impulse from a fresh and undisturbed body of air; and if
the vibration of the wing is limited to an arc of two feet, this by no
means represents the small force of action that would be obtained when
in a stationary position, for the impulse is secured upon a stratum of
fifty-eight feet in length of air 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.'

Thus Wenham, one of the best theorists of his age. The Society with
which this paper connects his name has done work, between that time and