any margin of lift-an impracticable degree of lightness. His mathematical training showed him how to evade this, though he never flew in fact. He showed the only way in which such a difficulty might be overcome, and at this early date gave the theorem which later emboldened Zeppelin and Lebaudy, the apostles of the enormous balloon, to work on such a vast scale. He saw that, when the volume of the ball is increased, not only is the lift' increased, but the amount of surface of the enclosing shell is not increased in proportion. Therefore, said he, without perhaps fully appreciating what other difficulties might be introduced by the stupendous dimensions which he was unwittingly contemplating, there must be some large dimension at which not only flotation, but flotation with a margin for lifting a man, can be secured. That the atmosphere would press in very heavily on the exhausted metal balloon, did not deter him, as he did not know how strong against collapse a metal balloon might have to be; and unhappily for him he did not know how to dispense with the vacuum by substituting for it in his balloon the light gas, hydrogen, or the light gas, hot air.

At the present day hydrogen is always used where the utmost lift is required; coal-gas, whose employment for balloons was introduced by Charles Green in 1820, is used where cheapness is of paramount importance. It is usual to say that 1000 cubic feet of hydrogen can lift 70 lb., and that 1000 cubic feet of coal-gas can lift 30 lb. ; and, as the cost of hydrogen has within the last year or two been much reduced, it is possible to say that, for equal lift in districts where hydrogen is a by-product of alkali works, even price is not in favour of coal-gas.

So long as directed flight had not been achieved, flight itself was an object; but to-day the object of flight is to reach a particular place, the way of the air being preferred because other routes are impeded by terrestrial obstacles. It is true that the reaching of a desired point by balloon can often be effected on a day which may be too windy for any aeroplane to start; but the return cannot be made, and even reaching the goal depends upon searching the sky for an air-current which travels appropriately. This search is made by rising and falling to get clear of one current which is unfavourable and

into another which is favourable. The guiding rule in our latitudes-so far as there is any rule for the fickle winds-is that at higher levels the wind comes from the right hand of the man who was facing it on the ground, and is much faster. Current-hunting cannot, however, be continued indefinitely; for each rise means loss of ballast and each descent loss of gas, and only a rare chance can bring one into and out of a vertical current at the moments desired without such loss.* Accordingly the duration of a balloon journey is limited by the amount of ballast carried, and its direction is determined by the wind; but both can be appreciably affected by the skill which pilots have acquired in foretelling the levels and speeds of helpful winds, without making wasteful excursions in a vertical plane. This skill it is which makes ballooning intensely sporting -sport of the most manly kind, not where the combatants are a fleeing animal and a rifle-wielding man, but the great elements which little man outwits.

If the balloon is to be self-propelled, an elongated shape and an equipment bearing some relation to the machinery of a motor-car is added, and we have the airship. We, who are now accustomed by pictures to the cigar shape of airships travelling with the bluff-end foremost (figs. 1 A and 1 B), have almost forgotten how recently the majority of men would have supposed that the more pointed end should be placed in front for easy penetration of the air, as it is in a bullet. In 1809 Sir George Cayley showed that he had an inkling of the modern view, and had proved that Newton's imaginary laws of gas movement lead to entirely wrong results. The experimental work carried out to find this best shape, though not yet complete, was brought to a high degree of advancement by the Advisory Aeronautical Committee in 1911, and through the laborious experiments made by J. Damon and W. Watts. The mass of knowledge so accumulated was soon extended to struts and wires; and this, with the recent encouragement given by Mr Alec Ogilvie, has not been wasted on the aeroplane constructor, whose fuselage,

* There exists a class of airship in which rising and falling vertically can be largely effected without loss of gas, by mechanical means; but no appreciable use of the device has been made anywhere but in England, with one exception. The inclined plane has been chiefly used for Zeppelins.

body-work, struts and tubes are to-day taking the semblance of the airship form.

Since a balloon when elongated requires to be stiffened to carry the weight of the engine and crew, a light steel girder or frame is often linked up to the gas bag (fig. 1B). This girder does not differ in principle or in the quality of material or type of accessories from the steel aeroplane fuselage sometimes seen in use. The aerial propeller, too, required at the outset the very calculations which we now use for aeroplanes, and thus helped forward the progress of that invention. Most important of all, there is a triplicate relation between the air-resistance of the craft at a particular speed, the power which a petrol engine will provide at a certain rate of rotation, and the power which the propeller can usefully absorb at the same speed; and this relation in the airship is analogous to that in the aeroplane-one of the many facts which made the study of the one an apprenticeship for the other. The neglect of the early airship in England is, more largely than we are aware, accountable for our backwardness in all aeronautics; while it is hardly recognised in France to what an extent the forward position of that country is due to the zeal of Giffard, Renard, Santos Dumont, de la Vaulx and other pioneers of the small dirigible balloon.

To-day the airship is not so much uninteresting as eclipsed in interest. That the study of it is not dead is testified to by large orders which are yet being placed in Germany and France for execution this year and next. There is no doubt.that money and brains are being devoted to this research abroad; and aeronauts, undeterred by disasters, have achieved singular success within the last few months, notably with the German airship Victoria Luise' and certain Parseval balloons. It is noteworthy that in twelve years of work no Zeppelin airship has sacrificed a single aeronaut; nor have our small English ones, with thousands of miles to their credit.

The bulk of the airship, the numbers required to launch it or reharbour it, its loss of hydrogen, involving heavy baggage trains to replace wastage, the insufficient proof of its ability to withstand mooring in the open, form a strong indictment against it. On the other hand, those who have journeyed in airships, as well as those who