172 SCIENTIFIC RECREATIONS. 



repeated. In the cases of ordinary echo, when the speaker waits for the 

 answer, he must place himself opposite the rock. If he stand at the side the 

 echo will reply to another person in a corresponding place on the farther side, 

 for the voice then strikes the rock at an angle, and the angle of reflection 

 is the same, as in the case of light. 



But if it should happen that there are a number of reflecting surfaces 

 the echo will be repeated over and over again, as at the Lakes of Killarney. 

 The Woodstock Echo, already referred to, and mentioned by several writers, 

 repeats seventeen syllables by day, and twenty by night. In Shipley there 

 is even a greater repetition. Of course the echo is fainter, because the waves 

 are weaker if the reflecting surface be flat. But, as in the case of the 

 mirrors reflecting light, a circular or concave surface will increase the 

 intensity. A watch placed in one mirror will be heard ticking in the other 

 focus. Whispering galleries carry sound by means of the curved surface. 

 Sir John Herschel mentions an echo in the Menai Suspension Bridge. The 

 blow of a hammer on one of the main piers will produce the sound from 

 each of the crossbeams supporting the roadway, and from the opposite pier 

 576 feet distant, as well as many other repetitions. 



Refraction of sound is caused by a wave of sound meeting another 

 medium of different density, just as a beam of light is refracted from water. 

 One sound wave imparts its motion to the new medium, and the new wave 

 travels in a different direction. This change is refraction. The sound 

 waves are refracted in different directions, according to the velocity it can 

 acquire in the medium. If a sound pass from water into air it will be bent 

 towards the perpendicular, because sound can travel faster in water than in 

 air. If it pass from air into water its force will cause it to assume a less 

 perpendicular direction, there being greater velocity in water. The velocity 

 in air is only 1,100 feet in a second in our atmosphere. In water sound 

 travels 4,700 feet in the same time. When the wave of sound falls upon a 

 medium parallel to the refracting surface there is, however, no refraction 

 only a change of velocity, not direction. 



When sound waves are prevented from dispersing the voice can be 

 carried a great distance. Speaking tubes and trumpets, as well as ear 

 trumpets, are examples of this principle, and of the reflection of sound. 



There are many very interesting experiments in connection with 

 Acoustics, some of which we will now impart to our readers. We shall then 

 find many ingenious inventions to examine, the Audiphone, Telephone, 

 Megaphone, and Phonograph, which will occupy a separate chapter. We 

 now resume. 



Amongst the experiments usually included in the course of professors 

 and lecturers who have a complete apparatus at their command, and which 

 at first appear very complicated and difficult, there are some which can be 

 performed with every-day articles at hand. There is no experiment in 

 acoustics more interesting than that of M. Lissajons, which consists, as is 

 well known to our scientists, of projecting upon a table or other surface, with 



