DISCOVERY AND INVENTION 



651 



upon the Atlantic fitted with telephone 

 transmitters attached to the thin iron skin 

 of the hull, away down in the hold, and 

 the receiving telephone on the bridge. 



On shore there are huge bells at light- 

 house stations making fog-signals under 

 water, and each steamer as it approaches 

 the coast can pick up these submarine 

 sounds at a distance of 10 miles. 



Here is a completed invention which 

 some patient observer has evolved from 

 just such little beginnings as those I have 

 described. 



I doubt whether you could hear a fog- 

 signal through the air at any such dis- 

 tance as that. The air is at best but a 

 poor conductor of sound, and many illu- 

 sions of hearing are possible. 



It is difficult in any case to tell the 

 exact direction of a sound in a fog. It is 

 possible, too, that you might have an 

 echo from the sails of a vessel, and you 

 would then be entirely misled as to the 

 direction of the signal station. 



Then, again, an island anywhere near 

 casts a sound-shadow upon the water. 

 The sound-wave striking the island is de- 

 flected up into the sky, and you would 

 have to go up in a balloon to hear it, and 

 it may not come down again to the sur- 

 face for a mile or two beyond the island. 

 A ship quite close to the island might not 

 hear the sound. The captain, knowing that 

 the fog-horn should be heard at least a 

 mile or two away, imagines himself to be 

 much farther off than he really is, and in 

 the midst of the fog he may become con- 

 scious of the presence of the land only by 

 actual contact with it. 



Then the transmitting qualities of the 

 air are subject to variations on account 

 of unusual atmospheric conditions. You 

 may be quite near a fog-signal station 

 and yet hear the sound so faintly that you 

 imagine it to be far away. You may even 

 get an echo from the clouds ; but then 

 you know you are subject to an illusion, 

 for the sound seems to come from the sky. 



Now, sounds can be transmitted 

 through the water to far greater dis- 

 tances than through the air, and atmos- 

 pheric conditions have no effect. 



I don't want to confine your attention 

 to inventions that already have been 

 made. I want to show you also that 



there is room for something new. We 

 don't know everything yet and the list of 

 possible inventions is not yet closed. 

 Take, for example, the case we have been 

 talking about, the transmission of sound 

 through water. 



EXPLORING UNDER THE SEA 



Three-quarters of the earth's surface 

 is under water and has not yet been ex- 

 plored, at least to any great degree. The 

 only way we have of reaching the moun- 

 tains and valleys at the bottom of the sea 

 is by sending down a sounding line and 

 bringing up a specimen of the bottom at- 

 tached to the sinker. It is no joke, how- 

 ever, to reach the bottom of the deep, 

 blue sea through one mile or even two 

 miles of water, and it takes several hours 

 to make a single sounding. Just think of 

 all the time and labor involved in merely 

 ascertaining the depth. 



Why should we not send down a sound 

 instead and listen for an echo from the 

 bottom. Knowing the velocity of sound 

 in water and the time taken for the echo 

 to reach the ear, we should be able to 

 ascertain the depth of the deepest part 

 of the ocean in less than four seconds in- 

 stead of more than four hours. Here is 

 something worth doing. It has never 

 been tried. I have suggested it a number 

 of times, and I will now pass on the 

 thought to you in the hope that some of 

 you may care to take it up. 



Suppose you are on one of those steam- 

 ers provided with transmitter hulls 'and 

 telephone ear-pieces, and you send down 

 a little piece of gun-cotton or other ex- 

 plosive material to a safe distance below 

 your ship and then explode it by an elec- 

 trical contact. The sound-wave from the 

 explosion will, of course, go down to the 

 bottom and then be reflected up again, so 

 that after a certain length of time you 

 should get an echo from the bottom. 



Not only should you be able to tell the 

 depth of the ocean by an echo from the 

 bottom, but you might perhaps learn 

 something of the nature of the bottom 

 itself. A flat bottom should yield a single 

 sharp return, whereas an undulating bot- 

 tom should yield a multiple echo, like 

 that heard when you fire a pistol among 

 hills. 



