There have been numerous unconfirmed reports concerning the use of _cellon_, a tough and yet completely transparent material, in the construction of aeroplanes on the German side, and occasional hints of new "invisible" machines were dropped now and then. The reports probably are based on some foundation of fact, but there is little to show that _cellon_ is used to any large extent by the Teuton forces. Samples of the material reached New York late in 1915, but the actual uses to which it was put were not known at the time. The tendency in recent months, especially on the western battle front, has been the "attack in squadrons," instead of the individual combats which made international heroes out of Boillot, Immelmann, Boelke, Warneford and Navarre. The squadron attack was first employed by the Germans in the Verdun operations. Previous to that time, only bombing expeditions had been undertaken en masse, as many as sixty aeroplanes taking part in a single attack. But actual aerial combat usually engaged only two or four aviators. Early in February of the second year of the war, several famous French aviators fell victims to the new mode of warfare. It seems that as soon as a machine would appear above the trenches in that section, six or more German machines would rise quickly and surround the Frenchman. Outnumbered and surrounded on all sides the French machines rarely got back safely to their lines, among the first to be lost being George Boillot, world-famous as an automobile racer. The German tactics at once were imitated and improved on by the allied forces, and by July, 1916, the French had perfected a system of defense which, paradoxically speaking, may be termed "air-tight." French aviation squadrons would be held in readiness at all times to repel attacks, and twenty machines usually were considered a "unit." At first sign of a hostile aeroplane approaching, ten French machines would rise at top speed to a height of 10,000 feet, while five minutes later the second ten would follow, rising to 5,000 feet. The attacking machine usually would be found at a height intermediate between the upper and lower French squadrons, both of which would attack the invader vigorously, and with highly satisfactory results.