76 INTERMINGLING OF UNDULATIONS. 



talk of children at play, the whistle of a distant locomo- 

 tive, the bark of a dog, the crowing of a cock, the chirp of 

 a cricket, and the faint tones of the bell in the village spire, 

 miles away. Though we can not well attend to all these 

 sounds at once, we can hear them all, and, if we select any 

 one to listen to specially, we can hear it distinctly and 

 clearly, showing that the undulations which produce it 

 come to us through the air undisturbed by the undulations 

 of all the rest, which, however, they must necessarily trav- 

 erse at every conceivable angle on the way. 



John had a curious opportunity to observe the phenom- 

 enon of undulations crossing each other without serious 

 interference one evening while he was with Lawrence in 

 London. It was in St. James's Park. 



There are several large parks in London where people 

 go for recreation and amusement. The nearest, and, in 

 some respects, the most attractive of these, is St. James's 

 Park. This park is smaller than any of the others, but it 

 is nearer the heart of the town, and so is more accessible 

 to large numbers of people. The queen's palace and gar- 

 dens are near it on one side, the houses of Parliament, and 

 Westminster Abbey, and the Horse Guards (the great head- 

 quarters of the army) on another, and the streets all around 

 it are lined with gay shops and elegant residences. 



In the park is a long and beautiful lake, crossed in the 

 middle by a suspension bridge. There are walks along 

 the margin of the lake, arid chairs for people who wish to 

 sit and rest, and beds and borders of flowers, and swans, 

 and ducks, and other kinds of swimming birds upon the 

 water, and on pleasant summer evenings the grounds are 

 full of ladies, and gentlemen, and children walking about 

 and amusing themselves in various ways. 



One evening, about an hour before the sun went down, 

 as Lawrence and John were walking together in one of the 



