But in spite of these advantages airships have very numerous counterbalancing disadvantages, so marked, indeed, that it seems a question whether, if the world decided to entirely use aëroplanes in their place, it would be much the loser. The principal disadvantages would seem to be: 1. The resistance of the gas-bag. 2. Danger of fire from close combination of petroleum motor and gas-containing envelope. 3. Danger of fire from self-electrification of surface. 4. Difficulties in the way of applying the propulsive screws in the most effective position. 5. Difficulties of making gas envelope gas-proof. 6. Great cost of airships. 7. The great amount of personnel needed for the manipulation of large airships. 8. Great liability of being destroyed by aëroplanes in war. 9. Insufficient power of quickly rising. 1. _The resistance of the gas-bag._ From a mechanical point of view it is in opposition to science to attempt aërial navigation by pushing such a large resisting surface as the envelope of an airship against the air. In navigating an airship against the wind, as the latter increases speed is diminished, until a limit is reached when the motive power will be unavailing. Thus there are weather limitations to the airship. Not that the aëroplane is unaffected by the weather. That also has its limits; but recent practice has shown that the proportion of days when aëroplanes can fly is considerably larger than those on which airships can venture forth from their sheds.