With such a noiseless conning-tower the submersible can cruise about on foggy nights, or when the waves are just high enough to make a disturbance on the surface, running with the top of the conning-tower open so as to secure good ventilation as long as possible, until the enemy is nearly within striking distance. As the target is approached the conning-tower must be closed, the protruding top sunk lower and lower in the water, and finally completely submerged, nothing appearing at the surface but the periscope tube just above the waves. With the aid of this instrument the target may still be seen distinctly, but the arc of vision is limited, and guessing the distance or rate of speed of the target is very difficult. Nevertheless, by estimating the distance before submerging, and knowing the rate of speed of his little craft, the submarine gunner may still get his range and find his target. If the waves are at all high, this is very difficult, as the water, slopping over the periscope, obscures the vision for several seconds at a time and is very distracting. But some experiments carried on during the summer of 1908 show that, even in broad daylight, it is no easy matter for a battle-ship to detect the approach of submarines until well within torpedo range, even when an attack is expected. In these experiments the United States cruiser "Yankee" in Buzzard's Bay was attacked by five submarines of the most recent type. The "Yankee" remained stationary expecting the attack, but to offset this disadvantage the crew was fully aware of the exact time that the attack was to be made. Indeed the officers of the cruiser had watched the submarines steam away until they disappeared. When twenty miles from the "Yankee" the five submarines submerged and headed for the cruiser, making observations at intervals by means of the periscope. The day was perfectly clear, and all on board the "Yankee" were keenly watching for the expected submarines. Yet the first intimation they had of the proximity of the diving boats was the striking of five torpedoes against the cruiser's hull. Each submarine had scored a bull's-eye. Not content with this success, the submarines repeated the attack from a nearer point, again scoring five hits before their presence was detected.