Hitherto we have described the rigid and semi-rigid types of air-ships. We have seen that the former maintains its shape without assistance from the gas which inflates its envelope and supplies the lifting power, while the latter, as its name implies, is dependent for its form partly on the flat rigid framework to which the car is attached, and partly on the gas balloon.

We have now to turn our attention to that type of craft known as a NON-RIGID BALLOON. This vessel relies for its form ENTIRELY upon the pressure of the gas, which keeps the envelope distended with sufficient tautness to enable it to be driven through the air at a considerable speed.

It will at once be seen that the safety of a vessel of this type depends on the maintenance of the gas pressure, and that it is liable to be quickly put out of action if the envelope becomes torn. Such an occurrence is quite possible in war. A well-directed shell which pierced the balloon would undoubtedly be disastrous to air-ship and crew. For this reason the non-rigid balloon does not appear to have much future value as a fighting ship. But, as great speed can be obtained from it, it seems especially suited for short overland voyages, either for sporting or commercial purposes. One of its greatest advantages is that it can be easily deflated, and can be packed away into a very small compass.

A good type of the non-rigid air-ship is that built by Major Von Parseval, which is named after its inventor. The Parseval has been described as "a marvel of modern aeronautical construction", and also as "one of the most perfect expressions of modern aeronautics, not only on account of its design, but owing to its striking efficiency."

The balloon has the elongated form, rounded or pointed at one end, or both ends, which is common to most air-ships. The envelope is composed of a rubber-texture fabric, and externally it is painted yellow, so that the chemical properties of the sun's rays may not injure the rubber. There are two smaller interior balloons, or COMPENSATORS, into which can be pumped air by means of a mechanically-driven fan or ventilator, to make up for contraction of the gas when descending or meeting a cooler atmosphere. The compensators occupy about one-quarter of the whole volume.

To secure the necessary inclination of the balloon while in flight, air can be transferred from one of the compensators, say at the fore end of the ship, into the ballonet in the aft part. Suppose it is desired to incline the bow of the craft upward, then the ventilating fan would DEFLATE the fore ballonet and INFLATE the aft one, so that the latter, becoming heavier, would lower the stern and raise the bow of the vessel.

Along each side of the envelope are seen strips to which the car suspension-cords are attached. To prevent these cords being jerked asunder, by the rolling or pitching of the vessel, horizontal fins, each 172 square feet in area, are provided at each side of the rear end of the balloon. In the past several serious accidents have been caused by the violent pitching of the balloon when caught in a gale, and so severe have been the stresses on the suspension cords that great damage has been done to the envelope, and the aeronauts have been fortunate if they have been able to make a safe descent.

The propeller and engine are carried by the car, which is slung well below the balloon, and by an ingenious contrivance the car always remains in a horizontal position, however much the balloon may be inclined. It is no uncommon occurrence for the balloon to make a considerable angle with the car beneath.

The propeller is quite a work of art. It has a diameter of about 14 feet, and consists of a frame of hollow steel tubes covered with fabric. It is so arranged that when out of action its blades fall lengthwise upon the frame supporting it, but when it is set to work the blades at once open out. The engine weighs 770 pounds, and has six cylinders, which develop 100 horse-power at 1200 revolutions a minute.

The vessel may be steered either to the right or the left by means of a large vertical helm, some 80 square feet in area, which is hinged at the rear end to a fixed vertical plane of 200 square feet area.

An upward or downward inclination is, as we have seen, effected by the ballonets, but in cases of emergency these compensators cannot be deflated or inflated sufficiently rapidly, and a large movable weight is employed for altering the balance of the vessel.

In this country the authorities have hitherto favoured the non-rigid air-ship for military and naval use. The Astra-Torres belongs to this type of vessel, which can be rapidly deflated and transported, and so, too, the air-ship built by Mr. Willows.