486 POPULAR LECTURES AND ADDRESSES. 



of a ship travelling over water. How is it that 

 it makes the wave ? Where was the ship when 

 it gave rise to the wave BCD in Fig. 48 ? Answer : 

 the portion BCD of the wave-pattern is due to what 

 the ship did to the water when the ship was at 

 E, the point E being at the same distance behind 

 C that the point A is in front of C ; when it was 

 at E it was urging the water aside, and the effect 

 of the ship pushing the water aside was to leave a 

 depression. Now suppose the ship to be suddenly 

 annihilated or annulled, what would be the result ? 

 The waves would travel out from it, as in the case 

 of a stone thrown into the water. Again suppose 

 the ship to move ten yards forward and then stop, 

 what would be the result ? A set of waves travel- 

 ling forward w r hile the disturbance that the ship 

 made by travelling ten yards remains. Now 

 instead of stopping, let the ship go on its course : 

 the wave disturbance is going its course freed 

 from the ship, and travels forward. When the 

 disturbance originated which has now reached 

 any point C, the ship was as far behind that 

 point C as it now is before it. Calculate out the 



