Aeronautics (Science of artificial flying)

Collier's New Encyclopedia (1921)

the science of artificial flying. The first ventures into this field date back to 1783. They were based chiefly upon the discovery by Cavendish of hydrogen gas, and his demonstration that it was lighter than air. Scientists were quick to grasp the idea that if containers of sufficient size and light weight could be secured and filled with hydrogen they would ascend. The first experiments in balloons, however, were carried on without the use of hydrogen gas. Stephen and Joseph Montgolfier, brothers, of Annonay, France, in 1783 made a paper balloon of a capacity of 700 cubic feet, which they fill filled with heated air from a fire beneath the bag. On June 5, 1783, this pioneer in aerial navigation rose to a height of a thousand feet. A little later, a French scientist, Charles, with Cavendish's discovery in mind, planned a balloon which should be filled with hydrogen gas instead of heated air. The first obstacle encountered was the difficulty of creating a perfectly airtight envelope. This was overcome by the discovery by two brothers named Roberts, that a coating of dissolved rubber over silk fabric would prevent the hydrogen gas from escaping. The test of this new balloon was made on Aug. 20, 1783, and the bag as soon as liberated rose to a great height, but because of too great inflation of the gas split asunder and fell to the ground. Hitherto the balloons had carried no passengers, but on Nov. 21, 1783, Pilâtre and the Marquis d'Arlandes sailed over the Seine and a large part of Paris in a balloon filled with heated air, remaining above the earth 25 minutes. A few days later, Dec. 1, an ascent was made by Messrs. Charles and Roberts in a balloon filled with hydrogen gas. In the two years ensuing, many flights were made, the most notable of which was that of Blanchard, the Englishman, who, accompanied by an American, Dr. Jeffries, crossed the English Channel from Dover and landed at Calais. Up to this time ballooning had been carried on as an adventure or a sport. Scientists intervened, and sought through this novel medium to ascertain scientific data that so far had been based only on conjecture. The first ascent with this object in view was made in 1804 from Paris. It was learned that as the balloon rose the air became drier and that at 23,000 feet the chemical composition of the air did not vary from that at the surface of the earth. The two most notable ascents recorded were those made by Glaisher and Coxwell in September of 1862 and of Berson SPHERICAL BALLOON - UNITED STATES and Suring, July 31, 1901. In the former ascent it was claimed that a height of seven miles was reached, but this was not susceptible of proof, because at 29,- 000 feet Glaisher lost consciousness. The ascent continued until Coxwell, who was almost paralyzed, opened the valve with his teeth, and the balloon finally landed with both passengers safe. More reliable figures as to altitude were those recorded by Berson and Suring, when they ascended from Berlin to a height of 6.7 miles. This is the greatest height that has ever been attained by men in a balloon, although it was exceeded by Major Schroeder of America who reached an altitude of 33,113 feet in an airplane. Within the last twenty-five years, many notable balloon flights have been recorded, some of them negotiated with great risk to the aerial adventurers. Up to the present century, the longest flight was that made by M. Goddard, who traveled the distance of 1,032 miles from Leipsic to Wilna in a little over twentyfour hours. In 1900 De la Vaulx covered the distance from Vincennes, France, to Korosticheff, Russia, 1,193 miles, in 35% hours. In 1912, Dubonnet and Dupont made a trip of 1,211 miles in the "Condor II." But a still longer flight was that of Rumpelmayer, who on March 24, 1913, completed a journey of 1,493 miles from Paris to the vicinity of Kharkoff, Russia. In America, Hawley and Post in the balloon "America" drifted 1,172 miles from St. Louis, landing in a dense Canadian forest. The great defect of the globular balloon that was recognized almost from the date of its invention was its inability to be propelled or steered. The huge machine was wholly at the mercy of the winds. It could rise by lightening its load; it could descend by releasing its gas; but it could not choose its direction of flight. No goal except that of altitude could be achieved. The devices that were originally employed to supply this need were such as to almost provoke a smile. Blanchard, the English aeronaut, tried to use oars and a rudder, but failed. Another device was the making of a large hole in the side of the balloon, through which the gas escaping was by reaction to force the balloon forward. What doomed all these contrivances to failure in advance was the spherical shape of the balloon. The surface offered to the wind was too great to be overcome. It was only when the shape of the balloon was changed that propulsion and direction became possible. The first dirigible balloon that could be called a success was the invention of Giffard, a Frenchman. He constructed a spindle-shaped bag that was 143 feet long, 39 feet in diameter, and had a capacity of 75,000 cubic feet. From beneath the bag hung a car that carried a 3-horse-power engine that drove a three-blade propeller at the rate of 110 turns a minute. A triangular sail at the end served as a rudder. The first trip had only a relative success, as the strong dent of Paris, though of Brazilian birth, made notable advances in the development of the dirigible and created a sensation " A SPHERICAL BALLOON AND BALLOON SHED USED IN THE UNITED STATES ARMY wind was inimical to the experiment, but Giffard demonstrated that he could steer his craft readily and attain a speed of from six to ten feet a second. He had h solved the problem of directed and propelled aeronautical voyaging. His work ELEVATING PLANES, ONE ON EACH SIDE OF BAG ABOUT 200 FT a little later by the ease and accuracy with which he circled the Eiffel Tower in Paris. In Germany, the famous aircraft that took its name from its indefatigable inventor, Count Zeppelin, was pushed to completion in 1900, RUDDER C-50 40F FUSELAGE TWO 120 H.P. UNION MOTORS SIX CYLINDERS EACH. ONE ON EACH SIDE OF FUSELAGE UNITED STATES NAVY DIRIGIBLE, TYPE C was greatly improved on by a compatriot, but it was not until 1910 that the first Renard. In 1884 he built the airship "La France," which was a pronounced success. His motive power was electricity, furnished by motor that weighed 220 pounds and had 9 horse power. In 1898 Santos-Dumont, a resia great passenger Zeppelin made its initial trip over Lake Constance and made aerial traveling a matter of ease and luxury and measurably of safety. Scientific study of temperatures at high altitudes has been facilitated. While the invention and perfecting of the balloon was a triumph of human ingenuity, it offered few of the difficulties that faced those who dreamed of navigating the air in a heavier-than-air machine. The very name seemed to defy the immutable law of gravitation. But the audacity of the idea acted as a stimulant rather than deterrent. For a centuries, men have tried to achieve the seemingly impossible. In the time of the Renaissance, Leonardo da Vinci who did so many things and did them ex- THE FIRST WRIGHT BIPLANE cellently, sketched out several devices, which were however ineffective, because of the lack of the motor power that succeeding centuries have supplied. In the 19th century a great deal of attention was devoted to the subject in England. Gayley , Henson, Wenham and Stringfellow supplied illuminating ideas that were afterward utilized, and contributions were made by Pénaud and Mouillard in France. Only, however, with the approach of the 20th century did these theories and experiments begin to give a real promise of success. Otto Lilienthal, a German inventor, demonstrated the principles of passive flight, the value of the arched wings and the pressure of air upon the wings at various angles. Had he not met with a fatal accident in 1896, he might have anticipated the Wrights. Pilcher, Chanute, and Montgomery elaborated and improved on Lilienthal's work. In the period between 1890 and 1903, aeroplanes were built by Langley in America, Ader in France, and Maxim in England, but though they represented marked advances on anything before achieved, they fell short of real success. Both Maxim's and Adler's machines were wrecked at their first trial. Langley did somewhat better. Congress had appropriated $50,000 to further his invention. He had constructed a tandem monoplane with 48 feet of wing spread and 52 feet in length. A 50-horseengine was employed. His experiments power were carried on over the Potomac river, but at each of its two trials the machine was wrecked and thrown into the river. While the defective launching apparatus was chiefly responsible for the failure, there was a reaction in public interest, no more money was furnished by Congress , and the project was generally ridiculed. It remained for the Wright brothers, two remarkable young men of Ohio, to achieve the actual mastery of the air in a heavier-than-air machine. During their boyhood days, they had been obsessed by the idea of flying, but their thoughts did not take a practical turn in that direction until 1896. Then they gave themselves up to experiment and study, and in 1900 began to try to fly at Kitty Hawk, N. C., where the sand dunes and wide spaces gave them ample room for tests, Nearly three years passed, however, years of tremendous labor and concentrated thought and indomitable resolution. Then, on Dec. 17, 1903, they actually flew in a light glider fitted with a small motor. Their first flights lasted only for from 12 to 59 seconds. But they had flownand the aeroplane was born. Even then they did not spread their success broadcast, and it was only in 1908, after five years more of hard work, that they made a flight in public. But such news travels fast, and the world of science and invention was immensely stirred by what was permitted to be known. A school of enthusiastic experimenters sprang up in France, and practical results were soon shown by Blériot, Farnam, Delagrange and others, who by applying what they had learned of the work of the Wrights, constructed machines that made actual flights. The A CURTISS FLYING-BOAT first officially recorded European aeroplane flight was made by Santos-Dumont in 1906. He flew a distance of 700 feet, remaining in the air for 21 seconds. In 1907 Farnam flew 2,500 feet in 52.5 seconds. In 1909, Blériot crossed the English Channel from Calais to Dover in 37 minutes. In 1908, the Wright brothers visited France and made flights of several hours' duration. They were showered with honors by the French Government and people. From that time on the art of flying took its place as an important factor in the world's life and progress. For the military history and uses of aeroplanes, see AVIATION IN THE WORLD WAR, below. There are three leading types of planes in general use. The monoplane, as its name implies, has one plane surface; the biplane has two, one above the other, and is by far the most common; the triplane has three, and its value lies in the greater load that it is able to carry. The monoplane has extreme speed because of its lightness and can climb more readily than the others, but its management demands proficiency and involves more hazardous risks. It is unexcelled for sport or racing, but its inability to carry heavy loads detracts from its practical value. The biplane can adapt itself to almost any conditions of air and weather, and its strength and carrying capacity have made it the most popular type. The triplane is as yet little used, but it can be depended upon as a cargo and passenger carrier because of its strength. A combination of aeroplane and boat is the hydroaeroplane, that can navigate either element. Some of them carry single floats and others double floats, for the purpose of alighting on the water. Strength is a prime requisite in their construction, in order to enable them to withstand the buffeting of the waves. The buoyancy of the floats, in order to insure safety from disaster, should be double the weight of the machine when it is loaded to capacity. During the World War the attention of the world was engrossed by the great conflict, and there was little time or opportunity for spectacular or long-distance flights. But with the cessation of the struggle there came in quick succession a series of endurance and distance contests that broke all previous records in the history of aviation. A class of airmen had been developed to whom danger was a part of the day's They work, and feats were attempted that of fog previously_would have daunted the most they daring. Prominent among these were off the transoceanic flights. The honor of when first having crossed the ocean in a were heavier than air machine fell to America. upside On May 16, 1919, the United States navy seaplane " NC - 4 " " hopped off " from were Trepassey Bay, Newfoundland, on the they first leg of a trip to Lisbon, Portugal. ing It made the distance to Horta in the Azores, 1,200 miles, in 15 hours and 18 were minutes. It reached Lisbon on May 27, injured and then flew from there to Plymouth, exploit England, making one stop at Ferrol, prize on May 31. The entire distance traveled airplane AILERONS ELEVATOR CONTROL WIRES ELEVATORS TAIL PLANE RUDDER CONTROL RUDDER WIRES DIAGRAM OF A BRITISH VICKERS - VIMY BIPLANE. THE FIRST NON-STOP FLIGHT ACROSS THE ATLANTIC was 3,925 nautical miles, and the total flying time was 57 hours and 16 minutes. But while America won honors for the first crossing of the ocean, a flight that was still more sensational, because it involved no stop on the way and because the hazards attending it were greater, was that made by Captain John Alcock, of England, who with one companion flew from St. John's, Newfoundland, to Clifden, Ireland, a distance of 1,960 miles, in 16 hours and 12 minutes. The trip was made in a Vickers-Vimy biplane with a Rolls-Royce engine. The trip was started on June 14, 1919, and from the beginning the conditions were the worst imaginable. The aviators had no light from sun or moon or stars. R34 R - 34, BRITISH DIRIGIBLE THAT MADE THE FIRST passed through alternate layers and cloud and snow. At times had to climb out and chip the ice their engine. There were periods they did not know whether they traveling in the customary way or down. They were at one time so near the surface of the sea that they in danger of being engulfed. But plunged on, and the following morntheir machine landed nose down in a bog at Clifden, Ireland. The aviators badly shaken up, but not otherwise . Alcock was knighted for his and received the " Daily Mail " of $ 50,000. He was killed in an accident, while flying over UPPER WING ENGINE PROPELLER BOTTOM AILERON LANDING WHEELS AN AIRPLANE OF THIS TYPE MADE Normandy in the December following. See ALCOCK, SIR JOHN. A notable attempt, that if it did not achieve success at least deserved it, was the flight of Hawker and Grieve. They started to cross the Atlantic in a Sopwith biplane May 18, 1919. The team flew from St. John's directly toward Ireland, but after traveling twelve and a half hours and making about 1,100 miles, the circulation system became clogged and they forced to were descend. Luckily they were picked up by a Danish steamer, and six days later were landed in England, where they had been given up as lost. The first dirigible to cross the Atlantic was the British airship "R-34." This 11 R34 BALLOON FLIGHT ACROSS THE ATLANTIC gigantic craft was 643 feet long, was driven by five engines of 275 horse power each and was capable of a speed of from 40 to 60 miles an hour. She left East Fortune, Scotland, on July 2, 1919, and landed at Mineola, N.Y., in 108 hours and 12 minutes flying time. The distance was 3,130 miles. Her return to England, July 9-12, was made in 74 hours tinental race, completing the round trip of 5,400 miles in 67 hours, 3 minutes and 40 seconds. In the spring of 1920, a successful flight was made by British aviators from Cairo to the Cape in Africa. On Dec. 10, 1919, Captain Ross Smith, an Australian aviator, landed at Port Darwin, Australia, thus winning the aeroplane race from London to Australia, FRAME STRUCTURE CONTAINS 19 GAS FILLED BALLONETS SILK COVER -639 FEET - - FORE GONDOLA CONTAINING NAVIGATION CABIN, ENGINE, WIRELESS. R 34. MIDSHIP ENGINE CARS CARRYING CAPACITY 30TONS FIVE 275 HORSE-POWER ENGINES FOUR GONDOLAS REAR GONDOLA TWO MOTORS GEARED TO ONE PROPELLER --- RUDDER DIAGRAM OF THE BRITISH DIRIGIBLE R-34 and 56 minutes. The total distance traveled in the round trip was 6,330 miles, and the time was 183 hours and 8 minutes. Other notable events in aviation in America were the New York-Toronto air race and the trans-continental race from New York to San Francisco and return. In the first-named contest, the winner was Lieut. B. W. Maynard as regards actual net flying time. He averaged more than two miles a minute for the total distance of 1,042 miles. The same aviator won the trans-convia India, which had been begun on Nov. 12. The altitude record was made by Major R. Schroeder of America, who rose to a height of 33,113 feet on Feb. 27, 1920. Aerial mail service has been established in the United States between important cities, and regular cargo and passenger service is maintained by air from London to Paris, Rome to Milan, and from Berlin to various cities of Germany. Aviation in the World War. - The conflict that convulsed the world for over four years differed from any that preceded it, in the use of aviation as an offensive and defensive arm of military service. The airplane had been demonstrated as really practicable for navigation of the air only since 1908, when the Wright brothers had given their tests in Paris. For some time after that, it was thought of chiefly in connection with sports, and realization of its great importance in war was slow in coming. Even when the military authorities of the various nations took the matter up, they thought of it A GERMAN "TAUBE" MONOPLANE chiefly as an aid in reconnoissance. It could go where human spies or scouts could not. No trenches or entanglements could hinder it from seeking out the location and movements of the enemy. But its enormous value for other work was apprehended dimly if at all. This was shown by the comparatively small number of planes possessed by the belligerents when the war started. Germany, the best equipped of all the warring nations in this respect, had not quite 1,000; England had only 250, and France had barely 200. As a scout, the airplane may be said have to met expectations. The movements and concentrations of the enemy were detected with a fair amount of success. It was not wholly and always reliable, however, especially as the war progressed, and both sides grew expert in camouflaging their movements and taking advantage of darkness for withdrawals and renewals of troops. Two notable failures of aerial observers occurred when the Germans were able to concentrate vast masses of men on Verdun sector in 1916 with the a French generals all at sea as to the direction from which the attack was coming, and again when Hindenburg was able to withdraw his men from the Arras salient in March, 1917, without the Allied aviators having learned of the movement. The reasons for these occasional failures can be readily understood. The aviator has to fly so high to avoid attack from anti-aircraft guns, or so fast to escape the attack of airmen, that his opportunities of obser. enemy vation are lessened. A height of less than 10,000 feet was considered unsafe, as anti-aircraft guns developed in range and accuracy during the progress of the war. Under such conditions which must often be added unfavorable , to weather , accurate observation was often impossible. Still, with all these handicaps the aerial service justified itself as a valuable observation arm of the service. At sea also its value was The seaplane soaring in the air could demonstrated. detect the wake of a submarine more readily than it could be seen from the deck of a vessel. The "mother ship," on which the seaplane could descend, from which it could rise, and where it could receive repairs and supplies, enabled them to opcrate a long distance off the coast and made them the "eyes of the fleet." They could give warning of the approach of a hostile squadron A THREE-INCH ANTI-AIRCRAFT long before it hove in view. They could announce to France and England, as they frequently did, the coming of raiding Zeppelins and airplanes. For photographic work, the airplane proved invaluable. The camera was truthful and never in a hurry. What the airman's eyes frequently could not see was recorded by the camera, to be carefully deciphered at headquarters later on. In this way, trenches and fortifications could be clearly traced, and operations could be intelligently based upon these records. At first the airplane was chiefly relied on by both sides for an aid to the artillery in directing its fire. But gradually it fell into disuse for this purpose. The height at which it had to fly made its directions often inaccurate and then too the increasedly skillful use of camouflage in concealing the gun locations of the enemy thwarted the observer's efforts. Gradually this use of the airplane was discarded, and the kite balloon took its place. The work of the observer stationed in the balloon was less spectacular than that of the aviator, but far more accurate, and in many respects more important. His work was not marred by haste. He had powerful glasses that gave him a radius of 10 miles. He had time to study a suspicious thicket or clump of trees, and see whether it really marked a concealed gun position. He could keep constantly in touch with his batteries by telephone. To be sure, his work was extremely GUN, MOUNTED ON A TRUCK hazardous. But he had to be a fatalist and remain quietly in his position. His balloon offered an alluring target for the enemy's long-range guns. Hostile aviators swooped down at the great bag and sought to puncture it with bullets. If an incendiary bullet penetrated the fabric and ignited the hydrogen gas, the balloon was doomed. Not more than 15 or 20 seconds would elapse before the explosion came. The observer's only salvation then was the parachute with which each was equipped. In the 17 days before the armistice was signed, the American army alone lost 21 balloons in this way, but in return our own aviators and artillery brought down 50 German balloons in the same period. When the war began, the Germans had about 100 kite balloons of the Drachen type. The Allies had practically none. They set to work, however, and eventually produced the Caquot balloon, which proved to have so many advantages over the Drachen that Germany herself finally adopted it. The Caquot balloon has a length of 93 feet, while its largest diameter is 28 feet. It has a capacity of 37,500 cubic feet of hydrogen gas, and this proves sufficient to lift the mooring cable, the basket, two observers and all necessary equipment to a height, if desired, of 5,000 feet. The lines are so curved as to offer the least possible resistance to the air. It is made of rubberized cotton cloth. It has lobes of rubberized fabric to act as rudders. When the wind blows, the lobes, which are attached to the rear third of the balloon, fill with wind. When the air is calm, the lobes hang loosely. The construction of the balloon makes it ride horizontally and almost directly above its moorings. It is released and drawn down again by a windlass mounted on a motor truck, so that it can be transported to any desired location. A spe- DIAGRAM OF A KITE OBSERVATION BALLOON, USED IN THE UNITED STATES cial feature of the Caquot is the location of the balloonette or air chamber within the main body of the gas bag. To separate it from the gas chamber, a diaphragm of rubberized cotton cloth is used. There is no air in the balloonette when it is first fully inflated at what is practically the ground level, but as the balloon ascends asc the wind blows win into the balloonette through a scoop placed under the nose of the balloon. This forces up the balloonette and compensates for the inevitable leakage of gas from the envelope. The average life of a kite balloon on an active war front was only fifteen days, but it did valuable work while it lasted. So anxious were the Germans to destroy them that they gave an aviator who brought one down a credit equivalent to the one bestowed for 1/2 planes destroyed. The scouting plane and the kite balloon represent the defensive feature of aviation. But it was soon discovered that the air service could also be made a formidable weapon of offense. Machine guns were furnished that were So synchronized that they shot through the blades of the propeller. Aerial squadrons were organized that wheeled and dove and rose in accordance with a system of tactics as precise as those on There were Homeric land and sea. battles in the sky, in which as many as forty or fifty planes might be engaged at once. Rewards were offered for those who brought down the greatest number of enemy machines and the coveted title of "ace" was bestowed upon the airman who had the attested destruction of five or more planes to his credit. Supremacy in the air was eagerly sought for by both sides, for it meant that one's own planes could hang over the enemy's front and watch his movements, while he was debarred from doing the same thing in return. Planes were constructed with armored protection to ward off the enemy bullets. Sometimes, instead of fighting with aerial competitors, a daring aviator would swoop down near the ground and rain machine-gun bullets on a marching detachment of the foe. The keen rivalry between the aerial enemies stimulated the invention of devices that would increase the effectiveness of the service. Chief among these was the wireless telephone, that enabled the aviator to 1 keep within speaking radius of his commander in the air and his ground station. At the beginning of the war, aerial supremacy resided with the Germans, but as the conflict progressed it gradually swung to the side of the Allies, so that PROPELLER. BULLETS PASS BY PROPELLER BETWEEN REVOLVING BLADES PLANE AN AIRPLANE MACHINE GUN, when the armistice was signed they had an overwhelming superiority in men and machines. Far overshadowing this phase of aerial warfare, however, was the bombing machine. These were first developed and used on a large scale by the Germans. The dropping of bombs on fortified places came well within the spirit of the articles of war. But Germany went further and dropped them upon the helpless civilian population of Paris, London, and other cities. The claim that these were fortified towns in the accepted meaning of the word was merely a pretext. Not even hospitals were spared in the savage warfare she adopted. The design was not merely to inflict a certain number of casualties, which after all could not be considerable, compared with the whole population, but shake the nerves and weaken the morale of the people back of the firing line. How greatly they failed of this effect is now a matter of history. At first, Germany relied for this work chiefly on her Zeppelins, of which more than a hundred were constructed during the war. But these giant dirigibles proved unsatisfactory. They were too unwieldy, were largely at the mercy of wind and weather, and offered too great a target for antiaircraft guns and the hosts of planes that rose in the air like a swarm of wasps to attack the huge craft with bombs and incendiary bullets. Grad- RELOAD MAGAZINE PLANE 00 SYNCHRONIZED WITH THE PROPELLER ually their use was abandoned as their vulnerability was demonstrated. Thirty at least are known to have been destroyed, and the great majority became unserviceable before the end of the war. The same fate overtook the majority of the Gross, the Parseval, and the Schuette- Lanz types of dirigibles. The bombing planes which replaced them had manifest advantages over their predecessors. They were speedy, less liable to be sighted by the enemy, and large enough to carry a heavy complement of bombs. Their military value was enormous in breaking up enemy bases and depots and preventing the concentration of troops. During eight days of the German drive in 1918, French airmen dropped 317 tons of bombs in the German lines, and produced a demoralization that greatly increased the effectiveness of Foch's counteroffensive. Airplane bombs are constructed with great care, and so shaped that they offer the least possible resistance to the air. They have fins on their tails to assure a perpendicular fall. They are carried on the planes either suspended under the wings or fuselage of the planes or carried in the fuselage itself. A special mechanism is employed to release them at the will of the aviator. He can release one or many at a time, according to whether he wants a salvo or just a SANDBAG PROTECTION AGAINST AIR RAIDS "trail fire." A small lever in the fuselage effects the release. Care must be exercised to release them alternately when they are carried beneath the wings so as not to disturb too much the equilibrium of the machine. He can drop them so as to explode or not, for sometimes an occasion arises when he must unload over his own lines. There are three distinct types of bombs - demolition, incendiary, and fragmentation. The demolition bombs have a light steel shell, and are filled with T. N. T. or some other explosive of great destructive power. They are used against heavy structures like depots, railways, and ammunition dumps. The charge is set off by a detonator, separated from the contents of the bomb by a pin. When the bomb is released, the pin is pulled out automatically, and the detonator slides into position to explode the bomb the instant it strikes. Some of the demolition bombs weigh a thousand pounds and carry five hundred and seventy pounds of explosive. Fragmentation bombs are designed on explosion to scatter showers of frag- They carry smaller charges, ments. because their walls are thicker. They are designed to be used against troops and are timed to explode when but a The few inches above the ground. 5-Vol. I-Cyc timing mechanism has to have an accuracy of less than a thousandth of a second. Incendiary bombs are intended to set fire to inflammable structures. They weigh about 40 pounds and contain a combination of chemicals that develops an intense heat. A sodium element makes it difficult to extinguish the fire, because sodium explodes when water is poured on it. The task of dropping a bomb so that it will hit the object aimed at is a difficult one, especially when the aviator relies exclusively on his own judgment and eyesight. Many things influence the fall of the bomb-the height above the ground, the rate of speed at which the plane is traveling, the air currents, and the shape of the bomb. It moves in a parabolic curve. When it is first released it moves almost horizontally, as it shares the motion of the plane. Then gravity asserts itself, the bomb gradually curves, and as the velocity increases assumes a perpendicular position. All this requires expert calculation, as the bomb has to be dropped some time before the aviator is directly above the object he desires to hit. To remedy miscalculations, sights were adjusted to the height, speed, and other conditions. When the two sighting points came in line with the target, the aviator could release his bomb with the probability that an accurate hit would be registered. When by these improvements the bombing plane had been practically perfected, it proved a most formidable weapon of offense. More and more the armies came to rely upon them as an effective and almost indispensable adjunct in large operations. Especially when the enemy was in retreat, were the bombing planes useful in harassing his flight and increasing his demoralization. The work of the Allied aviators during the retreat of the Crown Prince's army to the Vesle and Aisne in July and August of 1918, and later in the driving of the Germans from France and Belgium just prior to the armistice, can scarcely be overestimated. Air raids over cities during the war were initiated by the Germans. Paris and London were the principal objectives. In the raids on England by planes and Zeppelins, 5,511 persons were killed and injured, of whom 4,750 were civilians. The raids by Zeppelins numbered 51 and those by bombing planes 59. Paris suffered to a lesser degree, but still heavily. An incidental feature of one of the Paris raids was the suffocation of about three-score people who had taken refuge in a subway tube. The destructive power of the air raids was steadily lessened, however, as defensive measures were adopted and perfected. Airplane squadrons were kept in reserve at London and Paris, ready to ascend aloft at an instant's notice to repel the invaders. Antiaircraft guns of great range and accuracy brought down both Zeppelins and planes. Searchlights of enormous candle power swept the skies at night in every direction. Sirens were stationed on the tops of buildings so that their shrill