"It is this way, sir," responded John.  "The regular big biplane of the bomber type carries two propellers with an engine for each propeller. If one motor fails them when flying, about all the other is good for is to make a landing with.  By reason of the great lightness of our airplane one good 400 horse-power motor is all we need for pulling purposes.  But suppose this should fail, as any motor might do?  We could not continue, any more than the other fellow, and would have to volplane to the ground.  Again, suppose we wished to fly continuously more than twelve hours?  We could not do so, as such a steady run would heat the best motor and ruin it.  These two Liberty motors, which we have, overcome all these troubles.  Both are so arranged that a simple switch connects and disconnects either one with the propeller, and both can be put at work at the one time if needed in a bad storm.  If one stalls, the other can immediately be thrown in and a forced landing obviated.  Moreover, if we could get fuel when needed, with this arrangement I am safe in saying we could fly steadily day and night, resting one motor and working its mate, for a week or more."