THE CONQUEST OF TIME AND SPACE 



the American Lake boats have had it from the first; 

 but at the present time all boats are being so made. 

 At first these towers were made circular in form; but 

 it was found that towers of this shape made sufficient 

 splash in passing through the water to attract atten- 

 tion at a considerable distance on a still night. This 

 shape was abandoned, therefore, and a boat-shaped 

 one adopted. 



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 sur- 

 face, 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 ap- 

 pearing 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 sec- 

 onds at a time and is very distracting. But some ex- 

 periments 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 



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