The vulnerability of a captive balloon to the enemy’s fire has been tested by many experiments with variable results. One established fact is that the range of a balloon in mid-air is extremely difficult to judge, and, as its altitude can be very rapidly altered, it becomes a very difficult mark for artillery to hit. A few bullet-holes in the fabric of a balloon make but little difference, since the size of the perforation is very minute as compared with the great surface of material, but on the other hand, a shrapnel bursting just in front of it may cause a rapid fall. It is therefore considered prudent to keep the balloon well away from an enemy, and two miles are laid down as the nearest approach it should make habitually.
Besides being of use on land for war purposes, balloons have been tried in connexion with the naval service. In France especially regular trials have been made of inflating balloons on board ships, and sending them aloft as a look-out; but it is now generally contended that the difficulties of storing the gas and of manœuvring the balloon are so great on board ship as to be hardly worth the results to be gained.
A very important development of military ballooning is the navigable balloon. If only a balloon could be sent up and driven in any required direction, and brought back to its starting-point, it is obvious that it would be of the very greatest use in war.
From the very first invention of balloons the problem has been how to navigate them by propulsion. General J. B. M. C. Meusnier (1754–1793) proposed an elongated balloon in 1784. It was experimented on by the brothers Robert, who made two ascensions and claimed to have obtained a deviation of 22° from the direction of a light Dirigible balloons.wind by means of aerial oars worked by hand. The relative speed was probably about 3 m. an hour, and it was so evident that a very much more energetic light motor than any then known was required to stem ordinary winds that nothing more was attempted till 1852, when Henri Giffard (1825–1882) ascended with a steam-engine of then unprecedented lightness. The subjoined table exhibits some of the results subsequently obtained:—
| Year. | Inventor. | Length. | Diameter. | Contents. | Lifting Capacity. |
Weight of Balloon. |
Weight of Motor. |
H.P. | Speed per hour. |
| Ft. | Ft. | Cub. Ft. | ℔. | ℔. | ℔. | Miles. | |||
| 1852 | Giffard | 144 | 39 | 88,300 | 3,978 | 2,794 | 462 | 3·0 | 6·71 |
| 1872 | Dupuy de Lôme | 118 | 49 | 120,088 | 8,358 | 4,728 | 2000 | 0·8 | 6·26 |
| 1884 | Tissandier | 92 | 30 | 37,439 | 2,728 | 933 | 616 | 1·5 | 7·82 |
| 1885 | Renard and Krebs | 165 | 27 | 65,836 | 4,402 | 2,449 | 1174 | 9·0 | 14·00 |
| 1897 | Schwarz | 157 | 46 39 |
130,500 | 8,133 | 6,800 | 800? | 16·0 | 17·00 |
| 1900 | Zeppelin I. | 420 | 39 | 400,000 | 25,000 | 19,000 | 1500 | 32·0 | 18·00 |
| 1901 | Santos Dumont VI. | 108 | 20 | 22,200 | . . | . . | . . | 16·20 | 19·00 |
| 1908 | “République” | 195 | 35 | 130,000 | 3,100 | . . | . . | 80 | 30 |
| 1908 | Zeppelin IV. | 446 | 4212 | 450,000 | . . | . . | . . | 220 | . . |
Giffard, the future inventor of the injector, devised a steam-engine weighing, with fuel and water for one hour, 154 ℔. per horse-power, and was bold enough to employ it in proximity to a balloon inflated with coal gas. He was not able to stem a medium wind, but attained some deviation. He repeated the experiment in 1855 with a more elongated spindle, which proved unstable and dangerous. During the siege of Paris the French Government decided to build a navigable balloon, and entrusted the work to the chief naval constructor, Dupuy de Lôme. He went into the subject very carefully, made estimates of all the strains, resistances and speeds, and tested the balloon in 1872. Deviations of 12° were obtained from the course of a wind blowing 27 to 37 m. per hour. The screw propeller was driven by eight labourers, a steam-engine being deemed too dangerous; but it was estimated that had one been used, weighing as much as the men, the speed would have been doubled. Tissandier and his brother applied an electric motor, lighter than any previously built, to a spindle-shaped balloon, and went up twice in 1883 and 1884. On the latter occasion he stemmed a wind of 7 m. per hour. The brothers abandoned these experiments, which had been carried on at their own expense, when the French War Department took up the problem. Renard and Krebs, the Officers in charge of the War Aeronautical Department at Meudon, built and experimented with in 1884 and 1885 the fusiform balloon “La France,” in which the “master” or maximum section was about one-quarter of the distance from the stem. The propelling screw was at the front of the car and driven by an electric motor of unprecedented lightness. Seven ascents were made on very calm days, a maximum speed of 14 m. an hour was obtained, and the balloon returned to its starting-point on five of the seven occasions. Subsequently another balloon was constructed, said to be capable of a speed of 22 to 28 m. per hour, with a different motor. After many years of experiment Dr Wölfert built and experimented with in Berlin, in 1897, a cigar-shaped balloon driven by a gasoline motor. An explosion took place in the air, the balloon fell and Dr Wölfert and his assistant were killed. It was also in 1897 that an aluminium balloon was built from the designs of D. Schwarz and tested in Berlin. It was driven by a Daimler benzine motor, and attained a greater speed than “La France”; but a driving belt slipped, and in coming down the balloon was injured beyond repair.
From 1897 onwards Count Ferdinand von Zeppelin, of the German army, was engaged in constructing an immense balloon, truly an airship, of most careful and most intelligent design, to carry five men. It consisted of an aluminium framework containing sixteen gas bags with a total capacity of nearly 400,000 cub. ft., and it had two cars, each containing a 16 h.p. motor. It was first tested in June 1900, when it attained a speed of 18 m. an hour and travelled a distance of 312 m. before an accident to the steering gear necessitated the discontinuance of the experiment. In 1905 Zeppelin built a second airship which had a slightly smaller capacity but much greater power, its two motors each developing 85 h.p. This, after making some successful trips, was wrecked in a violent gale, and was succeeded by a third airship, which, at its trial in October 1906, travelled round Lake Constance and showed itself able to execute numerous curves and traverses. At a second series of trials in September 1907, after some alterations had been effected, it attained a speed of 36 m. an hour, remaining in the air for many hours and carrying nine or eleven passengers. A fourth vessel of similar design, but with more powerful motors, was tried in 1908, and succeeded in travelling 250 m. in 11 hours, but owing to a storm it was wrecked when on land and burnt at Echterdingen on the 5th of August. Subscriptions, headed by the emperor, were at once raised to enable Zeppelin to build another. Meanwhile in 1901 Alberto Santos Dumont had begun experiments with dirigible balloons in Paris, and on the 19th of October won the Deutsch prize by steering a balloon from St Cloud round the Eiffel tower and back in half an hour, encountering on his return journey a wind of nearly 5 metres a second. An airship constructed by Pierre and Paul Lebaudy in 1904 also made a number of successful trials in the vicinity of Paris; with a motor of 40 h.p., its speed was about 25 m. an hour, and it regularly carried three passengers. In October 1907 the “Nulli Secundus,” an airship constructed for the British War Office, sailed from Farnborough round St Paul’s Cathedral, London, to the Crystal Palace, Sydenham, a distance of about 50 m., in 3 hours 35 minutes. The weight carried, including two occupants, was 3400 ℔, and the maximum speed was 24 m. an hour, with a following wind of 8 m. an hour.
Thus the principles which govern the design of the dirigible balloon may be said to have been evolved. As the lifting power grows as the cube of the dimensions, and the resistance approximately as the square, the advantage lies with the larger sizes of balloons, as of ocean steamers, up to the limits within which they may be found practicable. Count Zeppelin gained an advantage by attaching his propellers to the balloon, instead of to the car as heretofore; but this requires a rigid framework and a great increase of weight. Le Compagnon endeavoured, in 1892,
