So much has been said and written concerning the Zeppelin airship, particularly in its military aspect, that all other developments in this field have sunk into insignificance so far as the general public is concerned. The Zeppelin dirigible has come to be generally regarded as the one and only form of practical lighter-than-air type of aircraft. Moreover, the name has been driven home with such effect that it is regarded as the generic term for all German airships.

These are grievous fallacies. The Zeppelin is merely one of a variety of types, even in Germany, although at the moment it probably ranks as the solitary survivor of the rigid system of construction. At one time, owing to the earnestness with which the advantages of this form of design were discussed, and in view of the fact that the Zeppelin certainly appeared to triumph when all other designs failed, Great Britain was tempted to embrace the rigid form of construction. The building of an immense vessel of this class was actively supported and it was aptly christened the "May-fly." Opponents of the movement tempered their emphatic condemnatory criticism so far as to remark that it MAY FLY, but as events proved it never did. The colossal craft broke its back before it ever ventured into the air, and this solitary experience proving so disastrous, the rigid form of construction was abandoned once and for all. The venture was not in vain; it brought home to the British authorities more convincingly than anything else that the Zeppelin was a mechanical monstrosity. The French never even contemplated the construction of such a craft at that time, estimating it at its true value, and the British failure certainly served to support French antagonism to the idea. Subsequently, however, an attempt at rigid construction was made in France with the "Spiess" airship, mainly as a concession to public clamour.

Even in Germany itself the defects of the Zeppelin were recognised and a decided effort to eliminate them was made by Professor Schutte in co-operation with a manufacturer of Mannheim named Lanz. The joint product of their ambitions, the Schutte-Lanz, is declared to be superior to the Zeppelin, but so far it has failed to justify any of the claims of its designers. This vessel, which also favours the colossal, is likewise of the rigid type, but realising the inherent dangers accruing from the employment of metal for the framework, its constructors have used wood, reinforced and strengthened where necessary by metallic angle-iron, plates, and bracing; this utilisation of metal is, however, carried out very sparingly. The first vessel of this class was a huge failure, while subsequent craft have not proved much more successful.

In fact, one of the largest German airships ever designed, L4, is, or rather was, a Schutte-Lanz, with a capacity of 918,000 cubic feet, but over 6,000 pounds lighter than a Zeppelin of almost similar dimensions. I say "was" since L4 is no more. The pride of its creators evinced a stronger preference for Davy Jones' Locker than its designed realm. Yet several craft of this type have been built and have been mistaken for Zeppelins owing to the similarity of the broad principles of design and their huge dimensions. In one vital respect they are decidedly inferior to their contemporary - they are not so speedy.

The most successful of the German lighter-than-air machines are those known respectively as the semi rigid and non-rigid types, the best examples of which are the Gross and Parseval craft. Virtually they are Teutonic editions of the successful French craft of identical design by which they were anticipated. The Lebaudy is possibly the most famous of the French efforts in this direction. The gas-bag has an asymmetrical shape, and is pointed at both ends, although the prow is blunter or rounder than the stem. The gas-bag comprises a single chamber for the inflating agent, the distended shape of the envelope being sustained by means of an air-ballonet. By varying the contents of the latter through the agency of a pump the tension of the gas in the lifting envelope can be maintained, and the shape of the inflated balloon preserved under all conditions.

Beneath the gas-bag is a long strengthened girder, and from this in turn the car is suspended. It is the introduction of this rigid girder which is responsible for the descriptive generic term of "semi-rigid." On the other hand the "non-rigid" type may be roughly described as a pisciform balloon fitted with propelling machinery, inasmuch as the car containing the driving machinery is suspended from the balloon in the manner of the car in the ordinary drifting vessel. So far as the French effort is concerned the Bayard-Clement type is the best example of the non-rigid system; it is represented in Germany by the Parseval class.

The Gross airship has been definitely adopted as a military machine by the German authorities, and figures in the "M" class. The "M-IV" completed in 1913 is the largest of this type, and differs from its prototypes in that it carries two cars, each fitted with motors, whereas the earlier machines were equipped with a single gondola after the French pattern. This vessel measures 320 feet in length, has a maximum diameter of 44 1/2 feet, displaces 13 tons, and is fitted with motors developing 450 horse-power, which is sufficient to give it a speed of 47 miles per hour. This vessel represents a huge advance upon its predecessors of this design, inasmuch as thelatter were about 245 feet in length by 36 1/4 feet in diameter, and displaced only six tons, while the single car was provided with a motor developing only 150 horse-power, the speed being 28 miles per hour. Thus it will be seen that a huge development has suddenly taken place, a result due no doubt to the co-operation of the well-known engineer Basenach. The "M-IV" is essentially an experiment and great secrecy has been maintained in regard to the trials which have been carried out therewith, the authorities merely vouchsafing the fact that the airship has proved completely successful in every respect; conclusive testimony of this is offered by the inclusion of the vessel in the active aerial fleet of Germany.

But it is the Parseval which is regarded as the finest type of airship flying the German flag. This vessel is the product of slow evolution, for it is admitted to be a power-driven balloon. Even the broad lines of the latter are preserved, the shape being that of a cylinder with rounded ends. It is the direct outcome of the "Drachen-Balloon," perfected by Parseval and Siegsfeld, the captive balloon which is an indispensable part of the German military equipment.

The complete success of the suspension system in this captive balloon prompted Parseval to continue his researches and experiments in regard to the application of power to the vessel, so as to induce it to move independently of the wind. The suspension system and the car are the outstanding features of the craft. It is non-rigid in the strictest interpretation of the term, although, owing to the incorporation of the steadying hollow "mattress" (as it is called by its inventor), the strength of the suspension system, and the substantial character of the car, it conveys an impression of great solidity. The thinnest rope, both manilla and steel, in the suspension system is as thick as a man's finger, while the car, measuring 30 feet in length by 6 feet in width, carried out in wood, is a striking example of the maximum of strength with the minimum of weight, being as steady and as solid as a boat's deck. The propellers are collapsible, although in the latest craft of this class they are semi-rigid.

The mechanical equipment is also interesting. There are two propellers, and two motors, each nominally driving one propeller. But should one motor break down, or motives of economy, such as husbanding of fuel, render it advisable to run upon one engine, then the two propellers may be driven by either of the motors.

The inventor has perfected an ingenious, simple, and highly efficient coupling device to attain this end, but to ensure that the propeller output is of the maximum efficiency in relation to the engine, the pitch of the propellers may be altered and even reversed while the engine is running. When one motor only is being used, the pitch is lowered until the propellers revolve at the speed which they would attain if both engines were in operation. This adjustment of the propeller pitch to the most economical engine revolutions is a distinctive characteristic, and contributes to the efficiency and reliability of the Parseval dirigible to a very pronounced degree.

Steering in the vertical plane is also carried out upon distinctive lines. There are no planes for vertical steering, but movement is accomplished by tilting the craft and thus driving the gas from one end of the balloon to the other. This is effected by the manipulation of the air-ballonets, one of which is placed at the prow and stem of the gas bag respectively. If it is desired to descend the gas is driven from the forward to the after end of the envelope, merely by inflating the bow ballonet with air by means of a pump placed in the car. If ascent is required, the after-ballonet is inflated, thereby driving the gas to the forward end of the balloon, the buoyancy of which is thus increased. The outstanding feature of the "Drachen-Balloon" is incorporated in the airship. This is the automatic operation of the safety valve on the gas-bag directly by the air ballonets. If these ballonets empty owing to the pressure of the gas within the envelope, a rope system disposed within the balloon and connecting the ballonets and the gas-valve at the top is stretched taut, thereby opening the gas-valve. In this manner the gas-pressure becomes reduced until the ballonets are enabled to exercise their intended function. This is a safety precaution of inestimable value.

The Parseval is probably the easiest dirigible to handle, inasmuch as it involves no more skill or knowledge than that required for an ordinary free balloon. Its movements in the vertical plane are not dissimilar to those of the aeroplane, inasmuch as ascent and descent are normally conducted in a "screwing" manner, the only exception being of course in abrupt descent caused by the ripping of the emergency-valve. On one occasion, it is stated, one of the latest machines of this type, when conducting experimental flights, absolutely refused to descend, producing infinite amusement both among the crowd and those on board.

The development of the Parseval is directly attributable to the influence and intimate interest of the Kaiser, and undoubtedly this represents the wisest step he ever made in the realm of aeronautics. It certainly has enabled the German military machine to become possessed of a significant fleet of what may be described as a really efficient and reliable type of dirigible. The exact number of military Parsevals in commission is unknown, but there are several classes thereof, in the nature of aerial cruisers and vedettes.

The largest and most powerful class are those known as the B type, measuring about 240 feet in length by 40 feet maximum diameter, of 223,000 cubic feet capacity, and fitted with two motorsand two propellers. This vessel carries about 10 passengers, can climb to a maximum height of approximately 8,500 feet, and is capable of remaining in the air for twenty hours upon a single fuel charge. While this is the largest and most serviceable type of Parseval designed for military duties, there is another, the A class, 200 feet in length with accommodation for six passengers in addition to the crew of three, which is capable of attaining a maximum altitude of 6,700 feet, and has an endurance capacity of 15 hours. This class also is fitted with twin propellers and motors. In addition there are the C and E classes, carrying from four to eight passengers, while the vedettes are represented by the D and F classes, which have a maximum altitude of 2,000 feet and can remain aloft for only five hours upon a single fuel charge. These smaller vessels, however, have the advantage of requiring only one or two men to handle them. The present military Parseval dirigible is made in one of these five standardised classes, experience having established their efficiency for the specified military services for which they are built. In point of speed they compare favourably with the latest types of Zeppelin, the speeds of the larger types ranging from 32 to 48 miles per hour with a motor effort of 360 to 400 horse-power.

So far as the French airships of war are concerned, the fleet is somewhat heterogeneous, although the non-rigid type prevails. The French aerial navy is represented by the Bayard-Clement, Astra, Zodiac, and the Government-built machines. Although the rigid type never has met with favour in France, there is yet a solitary example of this system of construction - the Spiess, which is 460 feet in length by 47 feet in diameter and has a displacement of 20 tons. The semi-rigid craft are represented by the Lebaudy type, the largest of which measures 293 feet in length by 51 feet in diameter, and has a displacement of 10 tons.

One may feel disposed to wonder why the French should be apparently backward in this form of aerial craft, but this may be explained by the fact that the era of experiment had not been concluded at the time war was declared, with the result that it has been somewhat difficult to determine which type would meet the military requirements of the country to the best advantage. Moreover, the French military authorities evinced a certain disposition to relegate the dirigible to a minor position, convinced that it had been superseded by the heavierthan-air machine. Taken on the whole, the French airship fleet is inferior to the German in point of speed, if not numerically, but this deficiency is more than counterbalanced by the skill and ability of the men manning their craft, who certainly are superior to their contemporaries in Germany, combined with the proved character of such craft as are in service.

The same criticism may be said to apply to Great Britain. That country was backward in matters pertaining to the airship, because its experiments were carried out spasmodically while dependence was reposed somewhat too much upon foreign effort. The British airships are small and of low speed comparatively speaking. Here again it was the advance of the aeroplane which was responsible for the manifestation of a somewhat indifferent if not lethargic feeling towards the airship. Undoubtedly the experiments carried out in Great Britain were somewhat disappointing. The one and only attempt to out-Zeppelin the Zeppelin resulted in disaster to the craft before she took to the air, while the smaller craft carried out upon far less ambitious lines were not inspiritingly successful. Latterly the non-rigid system has been embraced exclusively, the craft being virtually mechanically driven balloons. They have proved efficient and reliable so far as they go, but it is the personal element in this instance also which has contributed so materially to any successes achieved with them.

But although Great Britain and France apparently lagged behind the Germans, appreciable enterprise was manifested in another direction. The airship was not absolutely abandoned: vigilance was maintained for a superior type of craft. It was an instance of weighing the advantages against the disadvantages of the existing types and then evolving for a design which should possess the former without any of the latter. This end appears to be achieved with the Astra type of dirigible, the story of the development of which offers an interesting chapter in the annals of aeronautics.

In all lighter-than-air machines the resistance to the air offered by the suspension ropes is considerable, and the reduction of this resistance has proved one of the most perplexing problems in the evolution of the dirigible. The air is broken up in such a manner by the ropes that it is converted into a brake or drag with the inevitable result that the speed undergoes a severe diminution. A full-rigged airship such as the Parseval, for instance, may present a picturesque appearance, but it is severely unscientific, inasmuch as if it were possible to eliminateor to reduce the air-resistance offered by the ropes, the speed efficiency might be raised by some sixty per cent and that without any augmentation of the propelling effort. As a matter of fact Zeppelin solved this vexatious problem unconsciously. In his monster craft the resistance to the air is reduced to a remarkable degree, which explains why these vessels, despite all their other defects are able to show such a turn of speed.

It was this feature of the Zeppelin which induced Great Britain to build the May-fly and which likewise induced the French Government to stimulate dirigible design and construction among native manufacturers, at the same time, however, insisting that such craft should be equal at least in speed to the Zeppelins. The response to this invitation was the Spiess, which with its speed of 45 miles per hour ranked, until 1914, as one of the fastest dirigibles in the French service.

In the meantime a Spanish engineer, Senor Torres, had been quietly working out a new idea. He realised the shortcomings of the prevailing types of airships some eleven years ago, and unostentatiously and painstakingly set out to eliminate them by the perfection of a new type of craft. He perfected his idea, which was certainly novel, and then sought the assistance of the Spanish Government. But his fatherland was not adapted to the prosecution of the project. He strove to induce the authorities to permit even a small vessel to be built, but in vain. He then approached the French Astra Company. His ambition was to build a vessel as large as the current Zeppelin, merely to emphasise the value of his improvement upon a sufficiently large scale, and to enable comparative data concerning the two designs to be obtained. But the bogey of expense at first proved insuperable. However, the French company, decided to give the invention a trial, and to this end a small "vedette" of about 53,000 cubic feet displacement was built.

Although an unpretentious little vessel, it certainly served to emphasise the importance of the Torres idea. It was pitted against the "Colonel Renard," the finest ship at that time in the French aerial service, which had proved the fastest airship in commission, and which also was a product of the Astra Company. But this fine craft was completely outclassed by the puny Astra-Torres.

The builders and the inventor were now additionally anxious to illustrate more emphatically the features of this design and to build a far larger vessel. The opportunity was offered by the British Government, which had been following the experiments with the small Astra-Torres in France. An order was given for a vessel of 282,500 cubic feet displacement; in this instance it was ranged against another formidable rival - the Parseval. But the latter also failed to hold its own against the Spanish invention, inasmuch as the Astra-Torres built for the British authorities exceeded a speed of 50 miles per hour in the official tests. This vessel is still doing valuable duty, being attached to the British air-service in France.

The achievements of the British vessel were not lost upon the French Government, which forthwith placed an order for a huge vessel of 812,200 cubic feet capacity, equipped with motors developing 1,000 horse-power, which it was confidently expected would enable a speed of 60 miles per hour to be attained. Thus France would be able to meet the Germans upon fairly level terms, inasmuch as the speed of the latest Zeppelins does not exceed 60 miles per hour. So confident were the authorities that a second order for an even larger vessel was placed before the first large craft was completed.

This latter vessel is larger than any Zeppelin yet built, seeing that it displaces 38 tons, and is fitted with motors developing 1,000 horse-power. It has recently been completed, and although the results of the trials, as well as the dimensions of the craft have not been published, it is well known that the speed has exceeded 60 miles per hour, so that France now possesses the speediest dirigible in the world.

The Torres invention has been described as wonderful, scientifically perfect and extremely simple. The vessel belongs to the non-rigid class, but the whole of the suspension system is placed within the gas-bag, so that the air-resistance offered by ropes is virtually eliminated in its entirety, for the simple reason that practically no ropes are placed outside the envelope. The general principle of design may be gathered from the accompanying diagram. It is as if three sausage-shaped balloons were disposed pyramidally - two lying side by side with one super-imposed, with the bags connected at the points where the circular sections come into contact. Thus the external appearance of the envelope is decidedly unusual, comprising three symmetrical ridges. At the points where the three bags come into contact cloth bands are stretched across the arcs, thereby forming a cord. The suspension system is attached to the upper corners of the inverted triangle thus formed, and converges in straight lines through the gas space. The bracing terminates in collecting rings from which a short vertical cable extends downwards through a special accordion sleeve to pass through the lower wall of the envelope. These sleeves are of special design, the idea being to permit the gas to escape under pressure arising from expansion and at the same time to provide ample play for the cable which is necessary in a flexible airship.

This cable emerges from the envelope only at the point or points where the car or cars is or are placed. In the British airship of this type there is only one car, but the larger French vessels are equipped with two cars placed tandem-wise. The vertical cable, after extending downwards a certain distance, is divided, one rope being attached to one, and the second to the other side of the car. The two-bladed propellers are disposed on either side of the car, in each of which a 500 horse-power motor is placed.

The Astra-Torres type of dirigible may be said to represent the latest expression in airship design and construction. The invention has given complete satisfaction, and has proved strikingly successful. The French Government has completed arrangements for the acquisition of larger and more powerful vessels of this design, being now in the position to contest every step that is made by Germany in this field. The type has also been embraced by the Russian military authorities. The Astra-Torres airship has a rakish appearance, and although the lines of the gas-bag are admitted to increase frictional resistance, this is regarded as a minor defect, especially when the many advantages of the invention are taken into consideration.