to be employed; the proper disposition or inclination of its parts
becomes a question of the first importance. According as the
_turns_ of the screw are more or less oblique with respect to the
air they strike or the axis on which they revolve, more or less of the
resistance they generate by their rotation becomes _resolved_, as
it is technically expressed, in the direction of the intended course:
in other words, converted to the purpose in view, namely, the
propulsion of the Balloon.

Our limited space here again prevents us from entering into a detail
of the experiments by means of which the true solution of this
question has been arrived at, and the proper angle determined at which
the superficial spiral exercises the greatest amount of propulsive
force of which such an engine is capable. These experiments have been
chiefly carried on by Mr. Smith, the ingenious and successful adapter
of this instrument to the propulsion of steam vessels, for a series of
years, with the greatest care, and at a very considerable expense; and
the result of his experience gives an angle of about 67° or 68° for
the outer circumference of the screw, as that productive of the
maximum effect; a conclusion which is further verified by the
experiments of Sir George Cayley, of Mr. Charles Green, the most
celebrated of our practical aeronauts, and others who have employed
their attention upon the subject. This conclusion requires only one
modification, which ought to be noticed; namely, that in cases of
extreme velocity, the number of the angle may be still further
increased with advantage, until an inclination of about 73° be
obtained; when it appears any further advance in that direction is
attended with a loss of power. With these facts in view, the impinging
surface of the Archimedean Screw, in the model under consideration,
has been so disposed as to form, at its outer circumference, an angle