378 Royal Institution : — Prof. Tyndall on the Acoustic 



but sounds of low pitch generally, whether momentary or not, suffer 

 more from an opposing wind than high ones. We had on the 13th 

 another example of the powerlessness of heavy rain to deaden sound. 



On the 14th the maximum range was 10 miles, but the atmo- 

 sphere did not maintain this power of transmission. It was a day 

 of extreme optical clearness ; but its acoustic clearness diminished 

 as the day advanced. In fact the sun was in action. We proved 

 to-day that by lowering the pitch of the Canadian whistle its sound, 

 which had previously been inaudible, became suddenly audible. 

 The day at first was favourable to the transmission of the longer 

 sound-waves. After a lapse of three hours the case was reversed, 

 the high-pitched syren being then heard when both guns and horns 

 were inaudible. But even this state of things did not continue, so 

 rapid and surprising are the caprices of the atmosphere. At a dis- 

 tance of 5 miles, at 3.30 p.m., the change in transmissive power 

 reduced the intensity of the sound to at least one half of what it 

 possessed at 11.30 a.m., the wind throughout maintaining the same 

 strength and direction. Through all this complexity the knowledge 

 obtained on July 3 sheds the light of a principle which reduces to 

 order the apparent confusion. 



October 15 was spent at Dungeness in examining the perform- 

 ance of Daboll's horn. It is a fine instrument, and its application 

 was ably worked out by its inventor ; still it would require very 

 favourable atmospheric conditions to enable it to warn a steamer 

 before she had come dangerously close to the shore. The direction 

 in which the aerial echoes return was finely illustrated to-day, that 

 direction being always the one in which the axis of the horn is 

 pointed. 



The 16th was a day of exceeding optical transparency, but of 

 great acoustic opacity. The maximum range was only 5 miles. 

 On this day the howitzer and all the whistles were clearly over- 

 mastered by the syren. It was moreover heard at 3| miles with 

 the paddles going, while the gun was unheard at 2\ miles. With 

 no visible object that could possibly yield an echo in sight, the pure 

 aerial echoes, coming from the more distant southern air, were di- 

 stinct and long continued at a distance of 2 miles from the shore. 

 Near the base of the Foreland cliff we determined their duration, 

 and found it to be 11 seconds, while that of the best whistle-echoes 

 was 6 seconds. On this day three whistles, sounded simultane- 

 ously, were pitted against the syren, and found clearly inferior 

 to it. 



On the 17th four horns were compared with the syren and found 

 inferior to it. This was our day of greatest acoustic transparency, 

 the sound reaching a maximum of 15 miles for the syren, and of 

 more than 16 for the gun. The echoes on this day were continued 

 longer than on any other occasion. They continued for 15 seconds, 

 their duration indicating the atmospheric depth from which they 

 came. 



On October 18, though the experiments were not directed to de- 

 termine the transmissive power of the air, we were not without 



