ON THE PEOGEESS OF AEEONAUTICS. 
365 
sion, viz., that although water is about 800 times denser than 
air, yet the rarer atmosphere is capable of supporting the heavy 
bird, whilst the fish is about the same density as the element 
which it inhabits. They forget, however, that the currents to 
which the denser elements are liable, and with which the fish 
can cope with ease, are limited to eight, or at most to ten miles 
an hour, but where might the balloon be in a similar period ? 
Possibly eighty miles away. Let us suppose a boat under the 
same conditions as often appertains to a balloon, viz. in a 
current of twenty miles an hour. The river, let us say, is five 
miles across, and we can propel our boat at the rate of five miles 
an hour, as M. Dupuy de Lome has lately accomplished in 
France with a balloon. The boat is accordingly propelled to the 
other side in one hour, but in that time it has drifted twenty 
miles down stream. 
Now by powerful machinery it is just possible to propel a 
rigid construction like a boat against such a current with per- 
fect impunity, but to attempt the same thing with a balloon is 
an abandonment of all scientific principles. Any attempt to 
force the balloon unnaturally into the condition of the boat 
must be made at the risk of a burst-up. 
In the case of the boat, the thrust of the propeller is in the 
same line with the centre of gravity and displacement. In the 
case of a balloon the propeller works from a pendant car con- 
siderably below the centre of displacement. 
If it were possible, therefore, to propel the balloon at a rate 
which bore a more suitable proportion to the force of the cur- 
rents to which it is liable — let us say at the least twenty-five 
miles an hour — we may, without travelling with it to note the 
effect, just imagine the balloon anchored to the ground from 
the car, and a wind of twenty-five miles an hour blowing against 
it, which is precisely the same thing. The balloon would be 
forced into an inclination of about 60° out of the perpendicular. 
The dimensions of a balloon intended to carry a propeller 
with the men or machinery to work it, could scarcely be less 
than 60 feet in height by 50 in breadth, and taking the mean 
it would be equal to a sphere whose largest section would con- 
tain about 2,372 square feet. If we take two-thirds of this, 
viz. 1,581 square feet — because remember that it is a compres- 
sible sphere — we shall obtain a surface upon which the whole 
force of the wind at twenty-five miles an hour will be expended ; 
and at that rate, by Rouse and Smeaton’s tables, the pressure 
upon each square foot exposed would be 3*07 5 lbs., equal to a total 
force of 4,861 lbs., or upwards of two tons. We find, there- 
fore, the pressure to which a balloon of that capacity would be 
exposed were a motive power furnished capable of propelling 
such a balloon at twenty-five miles an hour ; and we learn also 
