The balloon material, which is india-rubber coated, has the peculiar property of becoming electrified in dry air. When rolled up or creased in any way it rustles, and gives out electric sparks, the latter being (as shown by the experiments undertaken by Professor Bonsteim and Captain Dele for the Berlin Aëronautical Society) clearly visible in the dark. Now, the lower parts of the material of which the gas-cells are composed would, owing to the height to which the airship had ascended (1,100 m.) and the release of gas from the valves, become creased or folded upon each other, and the rubbing thus produced would be quite sufficient to generate the electric sparks above referred to. Under ordinary circumstances, when the space between the gas-cells and the outer envelope of the airship is full of atmospheric air, continually renewed, as when it is in full flight, these sparks would be harmless enough, but when the ship is at anchor, as at Echterdingen, this is not necessarily the case. We know that the carefully made tissue of the Continental Caoutchouc Company resists the penetration of hydrogen very strongly, but some may have leaked through into the space between the cells and the outer envelope, while it seems very probable that when the mechanics opened the valves, and the long axis of the balloon became inclined, more hydrogen entered this space and an explosive mixture was formed. According to the description given by eye-witnesses, the explosion took place after the forepart of the vessel (dragging its anchor) struck the ground. The shock thus caused would have been transmitted to the creased and wrinkled gas-cells, and the tearing of the material, already in an electrified condition, might easily have generated sufficient sparks to detonate the explosive mixture. Again, in 1912, there was a repetition of this kind of disaster in the case of the destruction of another Zeppelin, the “Schwaben.” In this case the framework of the airship had got broken, being battered about in landing in an adverse wind. The india-rubber-coated bags were rubbed against each other, with the production of electric sparks. These either set fire to the gas issuing from one of the gas-bags or exploded the mixture of air and gas contained in the space between the gas-bags and outer covering of the airship. Perhaps it was on account of this accident that gold-beaters’ skin has sometimes been used for the gas containers of the Zeppelin airships. 4. _Difficulties in the way of applying the propulsive screws in the most effective position._