3U POPULAR LECTURES AND ADDRESSES. 



a horizontal plane because the vibrations are 

 vertical. 



Here is another mode of vibration. Let me 

 twist this spherule in the jelly as I am now doing, 

 and that will produce vibrations, also spreading 

 out equally in all horizontal directions. When I 

 twist this globe round it draws the jelly round with 

 it; twist it. rapidly back and the jelly flies back. 

 By the inertia of the jelly the vibrations spread 

 in all directions and the lines of vibration are 

 horizontal all through the jelly. Everywhere, miles 

 away that solid is placed in vibration. You do 

 not see the vibrations, but you must understand 

 that they are there. If it flies back it makes 

 vibration, and we have waves of horizontal vibra- 

 tions travelling out in all directions from the 

 exciting molecule. 



I am now causing the red globe to vibrate to 

 and fro horizontally. That will cause vibrations 

 to be produced which will be parallel to the line 

 of motion at all places of the plane perpendicular 

 to the range of the exciting molecule. What 

 makes the blue sky ? These are exactly the 



