MECHANICS. 65 
less than at the surface of the earth, care must be taken to fill the balloon to 
only three fourths of its greatest capacity ; in order, however, to cause it to 
appear equally stretched at all times, a second balloon is placed within the 
first, and about one fourth of its cubic contents. This second balloon is filled 
with air by a tube communicating with the car, but can be emptied through 
valves during the expansion of the gas. The balloon is, moreover, pro- 
vided with a warming apparatus, partly by heating the gas in the large 
balloon to about 60° R., to distend the balloon completely, and so bring to 
vear the entire force of ascension ; and partly to increase and diminish this 
ascending force at pleasure. This heating apparatus consists of a system 
of tubes, C, attached internally to the bottom of the balloon, and connected 
by a conducting tube with the heating apparatus or the boiler of the steam 
engine hereafter to be mentioned, so as to admit of being filled with steam 
or air. 2. The spar and sail work. To the balloon is fastened a light spar 
work, consisting of iron, covered externally with tin plate, and strengthened 
by braces and tight ropes. This rests against the spindle or middle column 
standing in the middle of the car, and consists of an iron frame covered 
with some air-tight material. In it is a strong spiral spring, which serves 
the purpose of weakening the shock caused by striking on the ground. In 
the spar work is a horizontal main-mast with the horizontal mainsail, D, D, 
whose two halves may be brought into any inclined position, to produce a 
change of horizontal and vertical direction according to the rules of navi- 
gation. 4H, H, are vertical sails for using the wind in change of vertical 
direction. 3. The car with the steam-engine. The former is fastened in 
the spar work, and provided beneath with strong spring buffers to weaken 
the shock in the descent of the vessel. It is divided by a floor into two 
compartments, the lower intended to serve as a coal, water, and freight 
room, the upper one for the passengers and engine; the latter a high 
pressure and of the rotating construction. 4. The motive apparatus to be 
driven by the engine, consisting of three spiral wind wheels GG, and the 
horizontal wind wheels, FF (jigs. 44, 46,47), the latter attached above 
the car in the middle of the plane of the mainsail. They consist of 
wings turning in a box, in the centre of which the wind enters, to be again 
driven out at some point of the circumference. The object of these 
‘horizontal wind wheels is to produce an artificial current in a determi- 
nate direction, which, acting upon the part of the horizontal mainsail 
presented to it, produces an oblique upward or downward motion. As far 
as known, this idea of the Pneumodrome has never been carried out on a 
large scale. 
I. Or tHe Motion or tur Arr.—PNeuMATICS. 
If any aeriform body be confined in a vessel, it must escape through a given 
aperture whenever it becomes more condensed than the air in the space to 
which the opening leads. A vessel used for containing any kind of gas, and 
from which the gas will stream forth with a certain rapidity on the appli- 
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