ened; the barometer, or corresponding apparatus
for telling the height ascended, or the upward or
downward course of the balloon; the map and
compass, for showing the direction of the voyage;
and the grappling-iron, tied to the end of a long
rope, for anchoring the balloon at the descent.
During his flight the aëronaut has at his dis-
posal the means of guiding his airship only in
an upward or downward direction, the motion of
translation being wholly dependent on the wind
by which it is borne. If he wishes to ascend, he
throws some of the ballast over the side of the
car, and if to descend, he pulls the valve-rope,
so that, the gas rushing by virtue of its specific
lightness through the passage made for it by
the open valve, the buoyant material may be
lessened. It is evident that the power of thus
directing his machine becomes more limited after
each discharge of ballast or gas, for, in each case,
there is an unrepaired loss of the means neces-
sary to it. In ordinary flights the mouth of the
balloon is left open, so that there is no danger
of explosion arising from the expansion of the
gas in the rarer regions of the atmosphere. The
gas most commonly used for balloons is coal gas.
The diffusion that takes place through the open
neck is inconsiderable during the few hours that
an aërial voyage lasts. Early aëronauts, who
kept their balloons closed, frequently ran con-
siderable risk by inattention to the valve when
the imprisoned gas demanded vent for its expansion.
Captive balloons, as the name signifies, are balloons which are held captive to the earth by means of a cable. The cable is usually arranged to be let out and hauled in by means of a windlass or drum operated by hand or by mechanical power. Captive balloons are much used in military operations and for amusement purposes, and to some extent for scientific observations. One of the largest captive balloons ever employed was at the Paris Exposition of 1878, where it made ascents with passengers from the Tuileries quadrangle. This balloon had a capacity of over 25,000 cubic yards and was made of canvas.
Dirigible balloons are balloons arranged with steering apparatus or propelling machinery by which the direction of their flight can he regulated at will. Various attempts have been made to design and operate dirigible balloons, but those which have attained the greatest success are: The experiments of M. Gaston Tissandier, made in 1883; the French army tests, made a year or so later, and, finally, the notable experiments of Count Zeppelin, made in the summer of 1900, and of M. Santos-Dumont. made in 1901. The balloon with which experiments were made by M. Tissandier was 91 feet long and 29 feet in diameter, built in the shape of a very thick cigar, with both ends pointed. The envelope was made of thin cloth covered with an impermeable varnish, and from it was hung by means of the usual netting and suspenders a car containing an electric battery supplying current to an electric motor which operated a screw propeller 9¼ feet in diameter and having two blades. A triangular silk rudder was fitted above the propeller in much the same relative position as the rudder of a steamship, and arranged so as to be operated from the car. The total weight of the propelling machinery, the car and the appurtenances, exclusive of 850 pounds of ballast, was 1200 pounds, while the balloon itself weighed 600 pounds. With the propeller making 180 revolutions per minute this balloon was able to maintain its position against a wind blowing 6.8 miles per hour, and when traveling with the wind to deviate to one side or the other with ease. The French Government balloon, whose construction was suggested by Tissandier's experiments, was designed by MM. Renard and Krebs on similar lines to, but somewhat longer in comparison with its diameter than, Tissandier's. Seven ascents were made with this balloon during 1884-85, with the following practical results: In five of the ascents the voyagers were able to return to their starting point, and in one instance a velocity of 13 miles per hour was attained independently of the wind. The airship in which Count Zeppelin made his notable voyages of 1900 consists of a row of seventeen balloons, confined like lozenges in a package, in a cylindrical shell 420 feet long and 39 feet in diameter, with pointed ends. These balloons serve to lift the structure in the air, where it is driven forward or backward by means of large screw propellers operated by benzine motors. A pair of rudders, one forward and one aft, serve to steer the "airship." The crew and passengers occupy two aluminum cars suspended forward and aft, below the body of the balloon shell. From these cars, which are connected by a speaking tube, all the machinery of the "airship" is operated. The "airship" is made to run on a horizontal or inclined plane by means of a weight, which can be moved back and forth, on a cable underneath the balloon shell. When the weight is far aft, the bow of the ship points upward and the movement is upward; and when the weight is far forward, the movement is downward, and when the weight is exactly in the centre of the ship, the travel is horizontal. The aluminum cars are each 20 feet long and 3½ feet high. The framework of the shell is aluminum wire covered on the top with soft ramie fibre protected by pegamoid, and on the bottom with light silk. The seventeen gas bags, made of a special cotton material, are all separate from one another, and there is a safety valve for each, although only four have outlet valves. The Daimler benzine engines, one in each car, are of 16 horse-power capacity each, and weigh 715 pounds each. The screw propellers, two for each engine, have four blades and are 3¾ feet in diameter. At the first trial of the Zeppelin airship on July 2, 1900, with five persons in the cars, it rose 1300 feet above Lake Constance and traveled 3¾ miles in 17 minutes in the direction desired. An accident to the sliding weight and to one of the rudders caused a descent to be made, which was accomplished with perfect ease. At a succeeding trial on October 17, the airship attained a height of nearly 2000 feet, and there remained poised for 45 minutes. It then made a series of tacks, and described a circle of about 6 miles circumference. The wind exceeded a velocity of about 7 miles per hour, and the airship made headway against this wind for a considerable distance. After remaining in the air for about one hour, the ship descended to the lake with great ease, and was towed to its shed. In steering, stability, and equilibrium the test was pronounced very successful.
In 1901, M. Alberto Santos-Dumont, a Brazilian gentleman resident in Paris, excited widespread interest through his experiments with a dirigible balloon. This aëronaut built his
