38 THE LAST CRUISE OF THE CARNEGIE 



regions to confirm this. Any attempt to discover the cause for 

 the Earth's permanent negative charge must be based upon a 

 knowledge of potential gradient. 



This automatic recorder gives us traces at about tenfold the 

 rate possible with the eye-reading apparatus used on former 

 voyages. It is also very sensitive to changes in the electric 

 conditions of the air, because ionium-collectors are used. Ionium 

 is an element which has the property of giving "air-molecules" 

 in its neighborhood an electric charge, thus turning them into 

 "ions." These ions acting as carriers facilitate the transfer of 

 electricity from the air to the instrument, and eliminate any lag 

 during rapidly changing conditions. 



We shall now walk forward on the port side of the quarter-deck 

 past the jaunty little dinghy hanging in its davits. The control- 

 room built alongside the companionway contains many essential 

 parts of our equipment. The time-measuring device for the 

 sonic depth-finder with its control-panel is located here. This 

 electric sounding-device, loaned by the United States Navy, is 

 made up of three important units, the oscillator, the micro- 

 phones, and the timing mechanism. A large steel-diaphragm 

 oscillator, set face downwards in the keel of the ship near the 

 stern, is put into periodic vibration by electromagnets and pro- 

 duces a sound-wave which is reflected from the ocean-bottom. 

 The echo is picked up by microphones set in the vessel's hull, 

 and carried to the head-phones of the observer, who sits at the 

 control-panel. An accurate time-measuring device gives us the 

 exact time-interval between outgoing signal and returning echo. 

 With this information we can easily calculate the depth, for the 

 velocity of sound in sea- water is known. It is roughly one mile 

 a second, depending however on the temperature and salinity. 

 But as these factors for each water-level are determined on board, 

 we are able to sound with an unusual degree of precision. For 

 example, the observer reports that it took two seconds for the 

 echo to return. This means that the sound-wave traveled about 

 two miles, and the sea is one mile deep. This is the underlying 

 principle, although actually the procedure is somewhat more 

 complicated. 



