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POPULAR SCIENCE REVIEW. 
character of that used in rockets, that have stored up in them 
the elements of great power. In the case of a rocket, the 
process of producing gas by chemical combination is actually 
employed to propel the rocket, though in a violent and wasteful 
form. What quantity of force might be stored up in a mass of 
composition which, with some means of direction, was not be- 
yond the power of a balloon to support, has probably not been 
investigated ; it is hardly likely to approach the amount neces- 
sary to produce satisfactory results, but it must be enormously 
beyond the force of a human arm. Then some form of hy- 
drogen gas might be used to give power by combining with the 
oxy dising agents, and so one part of the fuel might support the 
other ; but the bulk becomes increased. Supposing, however, 
for a moment, that a great horizontal velocity could be im- 
parted to a balloon, in its present form, held in the vertical 
direction only by the weight of the car, such an application of 
force to the latter would tilt it very much over to one side into 
a position depending on the relative magnitude of the forces 
in action ; so that, on this account, special provisions might 
have to be made. It is unnecessary, however, to proceed further 
in such speculations. It is sufficient to observe that, for efficient 
steerage, it is generally considered that two elements are neces- 
sary — the application of great power in a very light form, and 
a balloon with a rigid frame ; to which may be added that, for 
voyages of extended periods, an envelope much more impervious 
to the passage of gas than the silk hitherto used is essential. 
P.S. — The Plate (CXV.) is produced from a block kindly 
lent by the editor of “ The Engineer ” newspaper. 
