THE HAMILTON ASSOCIATION, 47 
the knowledge of the speed of sound enables one under certain 
circumstances to measure distances. You see the flash of a distant 
gun and some seconds later you hear the report—the number of 
seconds, 380, gives the number of yards distance. You see a 
flash of lightning and you count the seconds following—4%, or 
call it 5, seconds tell you that the storm is one mile distant and you 
are safe for the present. 
The amptitude of loudness is dependent on the density of the 
atmosphere in which the sound is started, not on that in which it is 
heard. The atmosphere on the top of Mount Blanc is about half 
the density of that on the level. Two guns of equal calibre fired 
one on the summit and one on the level would have very different 
effects. The one fired in the dense atmosphere below would most 
likely be heard above ; but the one fired in the lighter air above 
would start such feeble vibrations that they would be very unlikely 
to travel far. 
An interesting experiment can be shown by suspending a bell 
in an air pump with an attachment by which you can ring the bell. 
As the air is withdrawn the ringing sounds fainter and fainter until 
it becomes inaudible. If hydrogen gas is let into the bell it does 
not help the sound, because it is such a thin and very elastic gas 
that the air vibrations get no support to travel on. If the lungs are 
filled with hydrogen and the subject attempts to speak, the resultant 
voice is a mere squeak. Whether this is because the vocal chords 
starts vibrations in the light hydrogen and therefore sound weak, or 
the hydrogen is unable to excite more than a feeble vibration of the 
vocal chords is a matter to be decided by the laryngoscope. 
The velocity of sound in water is more than four times its 
velocity in air ; its velocity in iron, 17 times ; its velocity along the 
fibre of pine, 10 times. ‘The reason is that the elasticity of these 
substances as compared with their respective densities is vastly 
greater than the elasticity of the air compared with its density. 
As we have more experience of the travel of sound through 
air than through any other medium, we will consider the different 
conditions of air and their effect on the transmission of sound. As 
before noted, sound travels more quickly in warm air than in cold, 
the difference being, roughly, about one foot for one degree 
Fahrenheit. 
