Progress toward the modern airship has, as we have seen, been by short and laborious flights. The disappointments and disasters have been almost numberless. Endless patience, perseverance, and dauntless courage have been demanded. Moreover, in the past the would-be master of the air has needed very considerable resources. On account of a lack of funds many promising designs have come to no definite end. In the earlier days of flying the work of construction was done chiefly by men of leisure and means. Not till a comparatively recent date has the work been put on a commercial basis and done by large manufacturing firms. One of the chief difficulties to be overcome was to discover an object of sufficient strength to be driven through the air, and yet so light that it could displace more than its own weight of air. No very great difficulty was experienced in constructing the spherical balloon, for the sphere is, of course, the natural shape which any flexible envelope will take. No framework was needed to stiffen the flimsy covering of such a balloon. The sphere is, in itself, a natural shape, and it has no tendency to change. The distorting action upon it is that due to the weight of the car; but by using a large net bag, enclosing the whole balloon, this has been so spread that the distortion is very slight, and the natural shape not interfered with to a very appreciable extent. The great pressure of the air has, of course, constituted many difficulties. At sea-level the air pressure is 14·7 lbs. per square inch. A vessel containing a vacuum has therefore to be strong enough to support 15 lbs. on every square inch of its surface. To make the envelope of a balloon strong enough to contain a vacuum is impossible for the purpose. Too great weight would be required. It has been found that the best course is to fill the balloon with hydrogen, the lightest of gases. In this way the difficulty as regards pressure is overcome, for the hydrogen presses upwards as strongly as the air presses inwards. Stated in round figures, 1,000 cubic feet of hydrogen weighs about 5½ lbs., and the same quantity of air about 80 lbs. It has been found, then, that 75 lbs. represents the gross lifting weight, and that from it must be deducted the weight of the envelope to arrive at the desired lifting effect. With the increased size of the balloon many difficulties have been removed, for the lifting weight increases faster than the superficial area of the envelope. The contents of a sphere increase as the cube of a diameter, but the area grows only as the square of the diameter. Therefore, if you double the diameter of a balloon you increase its capacity and consequently its gross lift by eight times. Even if it should be necessary to increase the thickness of the fabric of which the balloon is made, there is still a good margin left in favour of the larger balloon.