But the aim has been to obtain something more than the ordinary spherical balloon, which simply drifts in the air-currents. Such a balloon is helpless as far as direction is concerned. It simply ‘goes with the wind.’ Its weight may be varied, but not its direction. The aim of the inventors of steerable balloons has been to overcome helpless drifting by means of propellers and rudders, and by various means designed to avoid loss of gas in ascending and descending. Inventors in time past found that it was no easy matter to drive a large spherical object of a light and flimsy construction through the air. With the huge area which a spherical balloon offers to the wind, it was found impossible to make any headway at all, except in perfectly calm weather, or with the wind behind. Consequently the steerable balloon took on an elongated shape, the nose growing more and more pointed, so that it could ‘cut’ the air. But now a fresh call arose for new ways and means of construction. The simple bag, which served in spherical form, was useless for the new design. A rigid framework of suitable lightness and strength was called for—an extremely difficult matter. Indeed, even in the case of a ship built for the sea there are troubles in this direction. ‘The water supports it all along, while the load which it carries is more or less in lumps, distributed irregularly from end to end. A ship in still water, without any attacks by storms from without, is in danger of breaking its back. If it be divided up into short sections some will be found to possess great buoyancy and little load, while others will be carrying loads far in excess of their buoyancy. The ship must therefore be strongly constructed, so that the lightly loaded parts may be able effectually to assist the heavily loaded parts. As great longitudinal stiffness is required in a ship as in a bridge. In fact, the modern ship is actually modelled upon a railway bridge. The method of construction which made the great liner of to-day possible was invented by I. K. Brunel, who got the idea from the Menai Straits Bridge of Robert Stephenson.’ Longitudinal stiffness is, then, an absolute essential to any structure of the kind now in mind. The buoyancy must be fairly constant from end to end, the cars being suspended at intervals. That is to say, it has been found that the necessary stiffness must be attained whereby the weight of the suspended cars will be distributed in due proportion to every part of the balloon, not simply to the parts immediately above. This has been attained by means of a cleverly constructed framework of aluminium, and on a line with this improvement have come a number of drum-shaped gas-bags, made of rubber fabric and placed in allotted spaces in the framework. A kind of keel has also been introduced beneath the frame, giving additional stiffness and keeping the airship from rolling, just as in the case of seafaring craft.