"We'll surprise John and Tom when we get back to the field," chuckled Paul. "They won't be looking for this addition to the crew of the Sky-Bird." "I'd say not," declared Bob, also chuckling. And indeed Paul's little hairy friend did create a lot of interest when they arrived beside the airplane, John and Tom both playing with him, for several minutes, and going into hilarious laughter at the funny antics of the weazened-faced creature, which looked so much like the wrinkled old mulatto from whom he had been purchased, that Paul said he should henceforth be called "Grandpa." They put the monkey in the cabin, and climbed in themselves, since all was in readiness for the departure. Night had fallen, but the sky was clear and moonlit. So there was no trouble, by helping matters with their searchlight, in hopping off and turning their head across the big Atlantic toward the shores of Africa. As the trade-winds were blowing quite stiffly in their faces, John, who was at the throttle, determined to mount high enough to overcome their most resistant effects. When at an altitude of about five thousand feet, he brought the Sky-Bird out horizontally, with her nose set by compass toward Freetown. Before they could reach this African seaport it would be necessary for them to travel considerably more than two thousand miles and meet whatever storms might develop. But all had such confidence in the capabilities of the Sky-Bird that none had any worries, fierce as some of the Atlantic storms were known to be. As they could no longer see the sea beneath them, owing to the darkness and fog which lay between, John had to rely entirely upon intuition and his compass to strike Freetown. Aerial navigation over immense bodies of water is similar to navigation on the seas themselves, except that the indispensable sextant of the mariner is of little use in the air, owing to the high speed of travel and the fact that allowances have to be made for the drift of the machine when side-winds are blowing--an extremely difficult factor to determine accurately. In side-winds the machine makes leeway in addition to its forward movement, and it is the ratio of one to the other which the successful pilot must work out correctly, especially when flying above clouds or when land features are unobserved. In this particular instance our boys were supplied with charts indicating the trend of all normal winds in each locality and their approximate force at various altitudes. Thus, by consulting his speedometer, John was able to figure out with a fair degree of certainty what allowances he should make from dead reckoning in order to strike their destination--or rather, we should say that Tom, as John's aid, did most of this figuring, for a pilot generally has his hands full in guiding his steed.