THE WOOLWICH BALLOON ASCENT. 
407 
its upward motion through the air, which is less and less dense as 
the height increases, the balloon must become more and more 
buoyant. This it does by the expansion of the gas as the 
pressure of the surrounding air becomes less, and thus the balloon 
becomes more completely inflated. Beyond a point, however, 
the safety of tbe balloon becomes jeopardised by excessive 
stretching, and so gas has to be allowed to escape, and any further 
ascent must be obtained by throwing out weight in some shape, 
in other words ballast. A certain quantity of this, however, 
must be reserved for the descent, which may be otherwise too 
rapid and violent. Hence it follows that the duration of a 
balloon voyage is greatly limited, and repeated variations in the 
height at which it soars are only obtained by taking up a quan- 
tity of extra weight and expending an extra quantity of gas. 
The horizontal motion of translation, on the other hand, may 
be said as yet to have defeated all efforts to control it. The 
balloon, from the instant it leaves the ground, becomes thoroughly 
committed to the winds ; so completely does it become incor- 
porated in the surrounding air, that no matter how violent and 
gusty a wind may be blowing, the occupants of a balloon, on 
quitting the earth, experience the sensation of being in a dead 
calm, and can only detect whether there is any wind by their 
motion over the earth. Wholly immersed in a surrounding 
medium, the balloon is totall} 7 deprived of any fulcrum such as 
a ship finds in the water, by which the direction of the force of 
the wind may be diverted, and its only means of escape from 
the power of any current of air is by rising above it or 
descending below it. 
To bring these facts to practice, it becomes necessary, in order 
to have any choice as to the probable course of a balloon, to 
select a day with the wind in the desired direction, and in this 
there may be great disappointment and deception. During the 
siege of Paris balloon voyages assumed an importance altogether 
without precedent. Science was called into play with its full 
powers, and nothing was spared to secure success. A map lias 
lieen brought out by M. Jules Gfodard, and officially endorsed, 
showing the result of 66 balloon voyages attempted from Paris. 
By this it appears that 51 only descended in France, 5 falling 
in Belgium, 4 in Holland, 2 in Prussia, and 1 in Norway, 
while 3 were altogether lost. Out of the above, 16 fell in the 
enemy’s lines, of which 5 were captured. These statistics, 
while they indicate what important work might be done by re- 
liable balloons, give more undoubted evidence as to their uncer- 
tain and unmanageable nature than could be shown by any 
theoretical discussion. The recent Calais ascent is only another 
instance of the same difficulty being experienced. 
In attempting to control and guide a balloon, then, Mr. 
