by means of Motion only. 269 
might just balance its weight, the machine would be suspended 
in the air. But it is obvious that a single vertical axis could not 
be properly turned by any moving power carried in a vehicle ap- 
pended from that axis; because such an appendage would soon 
acquire a rotatory motion in the other direction, This incon- 
venience, however, might be overcome by having ¢wo such axes 
turning in opposite directions; and their bulk again might be 
materially lessened by making the vanes of the one enter be- 
tween those of the other, similar to the teeth of two wheels. A 
more compact form, it is true, would be to haye the one axis 
hollow, and the other within it, with the one set of vanes above 
the other: but it is easy to perceive that unless the one were far 
above the other, the upper vanes would establish such a down- 
ward current of air on the lower, as to render them useless, or 
rather hurtful ; and indeed it is probable that even in the other 
form the aéronauts would always have abundance of, fresh air 
blowing downwards about their ears, 
Before speculating too far, it may be proper to observe that 
the whole scheme must still prove abortive, unless the apparatus 
could be constructed so light that a man, or other portable mov- 
ing power, might be able to do as much towards turning of the 
machine as to support himself and his own share of it. Could this 
be accomplished, I presume the giving it a direction through the 
air, even against a moderate wind, might be easily effected. For 
if the weight of the vehicle below, or the centre of gravity, were 
shifted so that the revolving axes might lean from the perpendi- 
cular a little to one side, the whole machine would forthwith en- 
deavour to wing its way toward that side of the horizon; and 
this apparatus presenting but a small surface to the wind, could 
withstand or move against it with little resistance compared to 
that on the inflated side of a bulky halloon. 
The difficulty of regulating a balloon so as always to maintain 
a certain height in the air, as well as the spirit of novelty and ad- 
venture, seem to have induced the generality of aérial navigators 
to keep at aconsiderable distance, greater perhaps than necessary, 
above the earta’s surface. This circumstance, however, is un- 
fortunately attended with various disady antages, none of the least 
of which is the greater rarity of the supporting medium. But 
the most serious disaster resulting therefrom, is the inevitable 
destruction of the aéronauts, if by any accident they experience 
a fall. Besides, should the wind suddenly rise or change, they 
may, though involuntarily, be wafted to the midst of the ocean, or 
dashed against the mountain’s brow, without either time to count 
their beads, or bid a final adieu to those they left behind. But 
the machine just described, could it be made to fly at all, might 
easily be regulated almost to skim along the surface, and by this 
means 
