ECHOES OF BATS AND MEN 



widely different circumstances in which the two classes 

 of echo-ranging systems are actually used. Bats are in- 

 terested in detecting small insects at a few feet or yards. 

 The user of an airborne radar wishes to locate objects on 

 the ground and other airplanes some miles away. Bats 

 use sound waves, while radar employs radio waves of 

 only sUghtly greater wave lengths. Bats maneuver very 

 rapidly, the whole sequence of detection, turning toward 

 an insect, intercepting, catching, and swallowing, all oc- 

 curring withm 1 second. In ordinary use of an airborne 

 radar, the operator sees a spot on his oscilloscope 

 screen, notes how it changes in position relative to his 

 own flight path, and then takes appropriate action. This 

 may vary all the way from a turn to avoid any danger of 

 collision, if the two airplanes are airhners, to a close 

 pursuit and firing of a machine gun or rocket at the 

 other plane if it is an enemy in time of war. In both 

 cases the whole operation may be accompUshed by a 

 man sitting in a darkened cabin looking only at spots 

 on his radar screen. The bat does it all within one sec- 

 ond, in the dark, with a brain smaller than the eraser 

 on a pencil. 



To make comparison a quantitative one, we can best 

 tabulate the important quantities which are known for 

 the two systems and on which we may base estimates 

 of their relative efficiencies. The table on page 123 gives 

 approximately the range of the radar and also its weight 

 and power requirements. An efficient system for echo- 

 location should detect the smallest possible objects at 

 the greatest possible distances and it should do so with 

 the least possible power and the lightest possible appa- 

 ratus. Bulky installations of whirling machinery may be 

 impressive at first glance, but unnecessary complexity 

 and power expenditure are actually signs of inefficiency. 

 With this in mind, let us set up an efficiency index, an 



122 



