Introduction 



charges. In some cases, the sound of the guns travelled 

 farther than that of any other instrument ; in some cases 

 the trumpet and siren were about equal, but in most cases 

 the siren did the best. But there is something besides 

 carrying power in the siren which makes it eminently well 

 qualified for sending out powerful vibrations and receiving 

 back the echo. 



Professor Tyndall tells us not only that the reflection of 

 sound from a solid body like a ship is very great, but 

 also that he got astonishing results from acoustic and in- 

 visible clouds : 



u On the i yth of October, at about 5 p.m., the air being perfectly 

 free from cloud, we rowed towards the Foreland, landed, and passed 

 over the seaweed to the base of the cliff. As I reached the base 

 the position of the Galatea was such that an echo of astonishing 

 intensity was sent back from her side ; it came as if from an 

 independent source of sound established on board of the steamer. 

 This ceased suddenly, leaving the aerial echoes to die away 

 gradually in silence." 



On some occasions the learned professor was greatly 

 puzzled. Sometimes at a distance of three miles he was quite 

 unable to hear the report of the guns or the sound of the 

 whistles or sirens, while on other days these could be 

 heard at a distance of nine or ten miles. On one occasion 

 they drew up as near as two and a half miles to the shore 

 and still the sounds were inaudible. Suddenly a cloud ob- 

 scured the sun, a dark shadow overspread the sea ; then 

 every instrument was heard, and, while the cloud was still 

 obscuring the sun, they withdrew to a distance of six miles 

 and practically all the instruments were heard. 



On another day while experimenting, the air was optic- 



xii 



