easily change their course by means of the moons, the fourth,

from which their car was named, being the one that they knew

would be of most use. Their tremendous speed showed them they

should have time for exploration on their arrival, and that they

would reach their destination sooner than they had expected. The

apergetic force being applied, as we have seen, only to the

Callisto, just as power in starting is exerted on a carriage or

railway car and only through it to the passengers, Ayrault and