It was found that through some accident the petrol tank had started a leak, and that the liquid was slowly oozing out during the flight, without giving the first indication until the engines failed. It was just the kind of an accident which necessitated the removal of the reservoir in order to make the proper repair. "Which is the fastest machine?" asked Alfred, while they were waiting for the return of the reservoir. "The Morane, by all means," replied the lieutenant. "I thought the Caudrons were the best?" remarked Ralph. "It depends on what you mean by _best_," answered the lieutenant. "The different machines are used for various purposes, and while speed is important, there are other things just as necessary." "What other things?" asked Alfred. "Well, bomb-dropping for one thing; then, some machines are better adapted than others for directing artillery fire, or for using machine guns," said the lieutenant. "The captain has a Caudron," said Alfred, "and he doesn't use a gun on it either." "No; the Caudron is admirably adapted for observations, and is used almost wholly in France for directing artillery fire," answered the lieutenant. "It seems to me that this machine is good for observing," said Ralph. "Yes; and it is used for that purpose, but it is now being put to use for photographing purposes,--that is, most of the Maurice Farman types are so used," answered the lieutenant. "Why is that?" asked Alfred. "For two reasons; first, because you will see we can get a good view downward; and, second, because it is the most stable machine in flight, the latter making it especially well adapted to take good views," replied the lieutenant. "Which are the largest ships?" asked Ralph. "The Voisin and the Breguet, by all means. They are able to carry heavy loads, and are used as bomb-dropping machines, as their fuselages are especially well adapted to carry and release the missiles," said the lieutenant. "But I saw a Voisin at one of the hangars which had a big gun on it," said Ralph. "Ah, that was the Voisin Cannon, which carries a 37-millimetre gun," said the lieutenant. "That is used for bombarding captive balloons and Zeppelins, if the latter should ever make their appearance." "A 37-millimetre gun?" repeated Alfred. "Yes; that's about one and a half inch in your measure," answered the lieutenant. Within an hour the reservoir was again in position, and there was no delay in again mounting into the air. "We might take a little trip due east, as far as Toul, and north of that point we may have the privilege of seeing some of the enemy," remarked the lieutenant, as they mounted the machine.