is was in fact the statement of the principles on which
fire-balloons were afterwards constructed and successfully sent up,
excepting that air heated by fire, instead of fire itself, was used.

Others who came after Albert of Saxony held the same theory, but they
all failed to reduce it to practice, and most of these men coupled with
their correct notions on the subject, the very erroneous idea that by
means of masts, sails, and a rudder, a balloon might be made to sail
through the air as a ship sails upon the sea. In this they seem to have
confounded two things which are dissimilar, namely, a vessel driven
through water, and a vessel floating in air.

The fallacy here may be easily pointed out. A ship is driven through
water by a body in motion, namely, wind, while its rudder is dragged
through a body comparatively at rest, namely, water; hence the rudder
slides against or is pushed against the water, and according as it is
_turned_ to one side or the other, it is _pushed_ to one side or the
other, the stern of the ship going along with it, and the bow, of
course, making a corresponding motion in the opposite direction. Thus
the ship is turned or "steered," but it is manifest that if the ship
were at rest there would be no pushing of the rudder by the water--no
steering. On the other hand, if, though the ship were in motion, the
sea was also flowing at the _same rate_ with the wind, there would be no
flowing of water past the ship, the rudder would not be acted on, and
the vessel could not be steered.

Now a balloon, carried by the wind, cannot be steered by a rudder,
because it does not, like the ship, rest half in one medium which is in
motion, and half in another medium which is at rest. There is no
sliding of any substance past its side, no possibility therefore of
pushing a rudder against anything. All floats along _with_ the wind.

If, however, the balloon could be made to go _faster_ than the wind,
then steering would at once become possible; but sails cannot accomplish
this, because, although wind can drive a ship faster than water flows,
wind cannot drive a substance faster than itself flows.

The men of old did not, however, seem to take these points into
consideration. It yet remains to be seen whether steam shall ever be
successfully applied to aerial machines, but this may certainly be
assumed in the meantime, that, until by some means a balloon is
propelled _faster than the wind_ through the at