62 Prof. J. Tyndall on the [Jan. 15, 



both these days the syren proved to be superior to the horns, and on some 

 occasions superior to the gun. 



On the 10th and llth, a gale having caused our steamer to seek safety 

 in the Downs, we made land-observations. The duration of the aerial 

 echoes was for the syren and the gun 9 seconds, for the horns 6 seconds. 

 The duration varies from day to day. We sought to estimate the influence 

 of the violent wind, and found that the sound of the gun failed to reach 

 us in two cases at a distance of 550 yards to windward, the sound of 

 the syren at the same time rising to a piercing intensity. To leeward 

 the gun was heard at five times, and certainly might have been heard 

 at fifteen times the distance attained to windward. The momentary 

 character of the gun-sound renders it liable to be quenched by a single 

 puff of wind ; 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 atmosphere 

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

 optical clearness ; but its acoustic clearness diminished as the day ad- 

 vanced. In fact the sun was in action. We proved to-day that by 

 lowering the pitch of the Canadian whistle, its sound, which had pre- 

 viously 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 gun 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 distance 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 main- 

 taining 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 performance 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 atmo- 

 spheric 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 overmastered 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 



