National Geographic : 1914 Jun
DISCOVERY AND INVENTION* BY ALEXANDER GRAHAM BELL you a rather startling question: Did you ever put your head under water and chuck two stones together to see what the sound is like? If you have never done that, try it, and you'll get a new sensation. I did it once, and it sounded as if a man were hammering for all he was worth at my very ear. I then took two tiny little pebbles and tapped them together quite lightly under water, and it sounded like a man knock ing at the door. It was rather startling to hear such a loud noise from such a slight cause. Of course, the question at once arose in the mind: How far off could we hear the sound? So I sent a boy a hundred feet up the beach with a couple of stones, directing him to strike them together under water. I then submerged my head, and I could hear the sound about as read ily as before. Well, I determined to test the maxi mum possible distance, and sent the boy across the bay in a boat to the other side, to a point at least a mile away from the place where I stood, and I followed him with field glasses to see that he carried out my instructions. I saw him land on the other side, take off his coat, roll up his sleeves, and go down to a little plank wharf on the shore rising only a few inches out of water. He lay down upon the wharf, face downward, and put his hands into the water, and I then knew he was making signals with these stones. Now, the question was: Could I hear him? Quietly and gently I went into the water at my side of the bay, submerged my head, and listened for all I was worth. Well, you know, the signals came per fectly clear and distinct, through more than a mile of water, to my ear. It was one of the most astonishing revelations of what could be done with water. You know if you look away in the distance at a man firing a gun you can see the flash, and after a time you get the report; the sound takes time to travel through the air. It goes about 1,1oo feet per second; but in the water it goes five times as fast as that-over 5,000 feet per second. Water is a much better conductor of sound than air. DO FISH SIGNAL TO ONE ANOTHER BY SOUNDS ? Reflecting upon these various experi ments, the thought occurred: If two little stones tapped together can be heard under water, why, every tiny lobster that snaps his claws must make an audible click. I wonder if there are creatures in the water that signal to one another by sound. Well, I had occasion to try it once. Bathing in the Grand River in Ontario a great many years ago, I put my head very gently under water and listened, and, sure enough, "tick, tick," came a sound like a grasshopper's chirrup, and a little while after that a chirrup on the other side. There were creatures under the water that were calling to one another. I don't know whether all fish make sounds or not, but there are some fish that certainly do. The drumfish on our coast drums away in the water so loudly that you can hear him while you are walking on the shore. It is also a significant fact that all fish have ears. Why should they have ears if there is nothing for them to hear? Of this we may be certain-that there is a whole world of sound beneath the waves waiting to be explored, perhaps by some of you. I have wanted you to see how one obser vation leads to another. Starting with a very small thing-the chucking together of two pebbles under water, and follow ing this up by other observations-we broaden our field of knowledge and reach generalizations of considerable magni tude as the resultant of numerous small thoughts brought together in the mind and carefully considered. *An address to the graduating class of the Friends' School, Washington, D. C., delivered May 22, 1914.