It blew steadily harder, and there was, as Frank noticed, a difference in the sea, for the flood stream was now setting up against them and was growing shorter and more turbulent. There was a smaller interval between the waves, which seemed to become steeper and less regular. They curled over and broke about the boat with a sound that reacted unpleasantly upon Frank's nerves, and he was thankful that he could, after all, see very little of them. The sloop's motion also changed. One moment she seemed to be moving almost slowly, and the next she swung up in a quick, savage rush, with her bows in the air and the white foam boiling high about her. Sometimes, too, there was a thud and a splash astern, and the decks were swept by a deluge of seething water. In the meanwhile the boys had contrived to light a lamp in a little box which held a compass, and they laid it on the thwart before Mr. Oliver, though, as he explained in a word or two, it was particularly difficult to steer an exact course in a sea of that kind. It was on the boat's quarter, that is, she was traveling with the wind almost behind her at a long slant across the course of the waves, but each time an extra big wave foamed up astern Mr. Oliver let her fall off and run right down wind with it to prevent its breaking on board. Frank wondered how he did it, for the seas were following them and it was quite dark, but Mr. Oliver had no need to look around. He had for guides the sound of the oncoming seas, the pull of the tiller, and the motion of the boat, and, besides, from long experience his brain worked sub-consciously. He did not pause to consider when the bows climbed out and the stern sank down in a rush of foam, and had he done so, in all probability he would have brought the big mainboom smashing over. To run a fore-and-aft rigged craft, and a sloop in particular, before a badly breaking sea, is a difficult and somewhat perilous thing, and the ability to do it comes only from long acquaintance with the water, and, perhaps, from something in the helmsman's nature. The boat sped on furiously, though they presently lowered the peak down to reduce the sail further, and by degrees Frank became conscious of an unpleasant nervous tension that seemed to sap away his hardihood. There was nothing to do in the meanwhile, but he felt that if he were called upon for any difficult or hazardous service he would find himself incapable of it. He was drenched and shivering, and he did not want to move. He only wished to cower beside Harry under the partial shelter of the coaming. This was, however, a feeling that other folks occasionally experience who go to sea in small vessels, which they have to grapple with and overcome. It is when there is no particular call on him, and he can only stand by and watch, that terror gets its strongest hold on the heart of a man.