On Aérial Navigation. 29 
to overcome the vis inmertie of the materials of which they are 
composed, and the gradually increasing resistance of the air. To- 
wards the termination of the waft, if the movement. be properly 
contrived, the momentum accumulated in these surfaces will pro- 
long the effective waft as much beyond the time when the effort 
of the first mover has ceased, as will exactly restore the power 
absorbed at the commencement of the action. Thus the whole 
power will have been expended on the resistance of the air, and 
consequently in propelling the balloon. 
There are several difficulties of construction which occur in 
rotative wafts; the chief of which are, giving firm support and 
communicating motion to the axis at the necessary distance it is’ 
obliged to be placed from the boat ; whereas in the wing waftage 
the hinge is on the solid frame of the boat. The wing con- 
struction likewise offers an advantage of great importance,-—that 
of providing, if properly managed, a safe descent in case of acci- 
dent to the balloon, ‘The chief advantage of the rotary move- 
ment is its uniform action. [ think either construction may be 
made effectual, but [ prefer the wing plan as the easiest for our 
first experiments. Mr. Evans may see in my early papers upon 
this subject, that revolving flyers had not escaped my attention: 
indeed, the first experiment I made upon the mechanical prin- 
ciples of aérial navigation, was successfully executed, though ona 
very small scale and by very simple means, upon this very plan*. 
Some very ingenious observations on the subject of aérial na- 
vigation are made by a correspondent in your Magazine for 
March 1817. In the third paragraph, respecting the means of 
vertical motion, the plan of condensing air into a second hal- 
loon is adverted to as woithy of particular attention, This plan 
of increasing the specific gravity by condensation, and lessening 
it again by the escape of thle condensed air, was one of the earliest 
suggestions of the balloon-makers; but, though founded on a true 
principle, is quite inefficient in practice. The elastic pressure 
of air increasing as its density, no cloth is able to bear the force 
required: for instance, if a cloth be capable of resisting a lineal 
tension of five hundred pounds to the foot, let a balloon twenty 
feet in diameter be constructed of this cloth; it will readily be 
found upon calculation, that only from seventeen to twenty pounds 
of additional air can be pumped into it before it would arrive at 
the proposed tension. Thus a huge impediment to motion 
would be added to the machine, besides the additional bulk ,of 
the supporting balloon necessary to carry the weight of this in- 
cumbrance, without gaining any efficient power to compensate 
for these disadvantages, 
* Nicholson’s Journal for November 1809, p. 172. 
In 
