Improvement has followed improvement. In some designs two light frames have been spread out from the main structure of the airship, each carrying a propeller. Frames have also been introduced at the back of the airship, thus giving four propellers in all—two forward and two aft. With these have come fins or planes, designed with the view to keeping the nose of the airship foremost to the wind. Moreover, groups of planes have been employed, lying in horizontal position but capable of movement, and making it possible to steer upward at both ends or at one only, as required. Whilst these structures, which led to the Zeppelin, were in course of preparation, other designs of importance were being made, which led by degrees to airships of the nature of the Parseval. In these designs there was no elaborate framework. The balloon portion was in one—a huge shape, stout in the middle with a pointed tail and rounded nose, and carrying triangular planes, placed horizontally. This strange shape, not unlike a fish, was maintained simply by the formation of the bag, distended by pressure of the gas. Difficulties as regards the car were overcome by long ropes, the car being suspended some distance below. The ropes were attached to the balloon at intervals, thus distributing the weight of the car throughout almost the full length of the balloon. Later came improvements which permitted the car of the airship to slide, so to speak, upon the suspending ropes, thus giving greater freedom to the action of the propeller. To the design were also added two smaller ballonets, inside the large one, carrying air-ballast. And by means of clever manipulation these bags made it easier to keep the airship at an even keel. This aim was also aided by a small horizontal plane or elevator placed beneath the bow. Underneath the stern was hung a vertical plane, to the end of which the rudder was hinged. The motor was in the car, and drove two propellers, supported upon a framework, between the car and the balloon. These craft gradually grew to about 300 feet in length, and about 50 feet in diameter at the thickest parts. Other designs, which led to the Astra-Torres, an airship of French origin, had a balloon of ‘trefoil’ shape. The car was hung low, as in other models of the kind, and was distributed by a number of wires, some of which passed into the balloon itself and were attached inside. Indeed, it was this mode of attaching the car that led to the trefoil shape. Two planes were attached to the rear, and two elevators and the rudder were placed beneath the rear end. In another fairly successful design of a similar nature a long girder ran underneath the balloon, supported by wires from the balloon, the car being attached to the centre, thus distributing the weight throughout the whole length of the balloon.