2o5 



NATURE 



YJan. 15, 1874 



ON THE MOTION AND SENSATION OF 

 SOUND* 

 Lecture I. 



T T is needless for me to say to the ladies and gentlemen 

 ■*■ who honour these lectures with their presence, that 

 they are intended more especially for the instruction of 

 boys and girls. As in all other cases where it has fallen 

 to my lot to teach others, I shall endeavour, while avoid- 

 ing superficiality, to strip the subject of all unnecessary 

 difficulty, and of all parade of learning, and to present it 

 in simplicity and strength to the youthful mind. 



The title of the lectures is, The Motion and Sensation 



of Sound. Now every boy knows what I mean when I 

 speak of the sensation of sound. The impression, for 

 example, of my voice at the present time upon the organ 

 of hearing is the sensation of sound. I5ut what right have 

 I to speak of the motion of sound ? This point must be 

 made perfectly clear at the beginning. 



For this purpose I will choose from among you a repre- 

 sentative boy, or allow you to choose him, if you 

 prefer doing so. This 1)oy, whom you may call Isaac 

 Newton, or IMichael Faraday, will go with me to 

 Dover Castle, make the acquaintance of the general com- 

 manding there, Sir Alfred Horsford, and explain to him 

 that we wish to solve an important scientific problem. 

 He is sure to help us : he will lend us a gun, and an 





r*» 



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intelligent artilleryman ; and we will make arrangements 

 with this man to fire the gun at certain times during the 

 day. We set our watches together ; and now, before 

 quitting him, we ask the artilleryman to fire one shot. 

 We are close at hand, and we observe the flash 

 and listen to the sound. There is no sensible interval 

 between them. When we stand close to the gun flash 

 and sound occur to.;ether. 



Well, we quit the artilleryman, warning him to fire at 

 the exact times agreed upon. Let us say that the first 

 shot is to be fired at 12 o'clock, the second at 12.30, and 

 so on every half hour. We quit our artilleryman at half- 



past eleven, descend from the castle to the sea-shore, 

 where a small steamer is awaiting us. We steam out a 

 little better than a mile from the place where we have left 

 the artilleryman ; and now we pull out our watches and 

 wait for 12 o'clock. Newton at length says, " In 

 exactly half-a-minute the gun ought to fire ;" and, sure 

 enough, at the exact time agreed upon, we sec the flash of 

 the gun. But where is the sound which occurred with the 

 flash when we were on shore .'' We wait a little, and pre- 

 cisely five seconds after we have seen the flash we hear 

 the explosion ; the sound having required this time to 

 travel over a little better than a mile. 



We now steam out to twice this distance and wait for the 

 12.30 gun. We see the flash, but it requires ten seconds 

 now for the sound to reach us ; we treble the distance, it 

 requires fifteen seconds ; we quadruple the distance, and 

 find the sound requires twenty seconds to reach us. And 

 thus, if the day were clear, we might go quite across to 

 the coast of France and hear the gun there. In all cases 

 we should find that the flash appeared at the precise time 

 agreed upon with the artilleryman, which proves that 

 light reaches us in so short a time that our watches fail to 

 give us any evidence that the light requires any time at 

 all to pass through space, while the sound reaches us later 



* Royal Institution Christmas Lectures, 1873—4, by Professor Tyndall, 

 D.C.L., LL D., F. R. S. These lectures have not been written out, 

 much less intended for publication. At the request of our Reporter, Dr. 

 Tyndall has consented to their appearance in Nature. 



and later the farther we go away. I think these experi- 

 ments give us every right to speak of the " Motion of 

 Sound." 



But they also inform us how the velocity of sound has 

 been actually determined. The most celebrated experi- 

 ments on this subject have been made in France and 

 Holland. Two stations were chosen ten or twelve miles 

 apart ; guns were fired at each station, and the interval 

 between the flash and the report was accurately measured 

 by the observers at the other station. In this way it was 

 found that when the air is at the temperature of freezing 

 water, the velocity of sound through it is 1,090 feet a 

 second. On difi'erent days we should find it travelling at 

 different speeds — as the weather grows warmer the sound 

 is found to travel faster. 



But I must not let you go with the idea that light re- 



