of 68° with the axis of revolution, gradually diminishing as it
approaches the centre, according to the essential character of such a
form of structure.

The novelty of the application of this instrument to the propulsion
both of ships and balloons, suggests the propriety of a few more
explanatory remarks to elucidate its nature and meet certain
objections which those who are ignorant of its peculiar qualities are
apt to raise in respect of it.

Previous to the adoption of this particular instrument, various
analogous contrivances had been resorted to in order to produce the
same effects. Of these, examples are afforded in the sails of the
windmill, the vane of the smoke jack, and of more modern introduction,
the _propellers_ designed by Mr. Taylor for the equipment of
steam-boats, and which Mr. Green has availed himself of to shew
the effect of atmospheric re-action in directing the course of the
balloon. Now all these and similar expedients are merely modifications
of the same principle, more or less perfect as they more or less
resemble the perfect screw, but all falling far short of the efficacy
of that instrument in its primitive character and construction. The
reason of this deficiency can be readily accounted for. All the
modifications alluded to, which have hitherto been applied to the
purposes of locomotion, are adaptations of _plane_ surfaces.
Now it is the character of _plane_ surfaces to present the
same angle, and consequently to impinge upon the air with the same
condition of obliquity throughout. But the _rate_ of revolution,
and consequently of impact, varies according to the distance from the
axis; being greatest at the outer edge, and gradually diminishing as
it approaches the centre of rotation, where it may be supposed to be