Editorial Miscellany. 187 



coincided, a puiF escaped; when they did not coincide, the current of 

 air was cut off. In this way impulses were imparted to the air, and 

 sound-waves generated. The siren has been greatly improved by 

 Dove, and specially so by Helmholtz. Even in its small form, it can 

 produce sounds of great intensity. 



" In the steam-siren, as in the ordinary one, a fixed disk, and a rotat- 

 ing disk are employed, but radial slits are used instead of circular 

 apertures. One disk is fixed vertically across the throat of a conical 

 trumpet 16i feet long, 5 inches in diameter where the disk crosses it, 

 and gradually opening out till at the other extremity it reaches a 

 diameter of two feet three inches. Behind the fixed disk is the rotat- 

 ing one, Avhich is driven by separate mechanism. The trumpet is 

 mounted on a boiler. In our experiments, steam of 70 lbs. pressure 

 was for the most part employed. Just as in the ordinary siren, when 

 the radial slits of the two disks coincide, and then only, a strong puff 

 of steam escapes. Sound-waves of great intensity are thus sent 

 through the air, the pitch of the note dependmg on the velocity of 

 rotation." 



To these instruments were added three guns — an 18-pounder, a 5^- 

 inch howitzer, and a 13-inch mortar. 



The comparative tests to which these instruments were subjected at 

 first resulted in favor of the guns, which were heard at a distance of 

 nearly ten miles, while under other atmospheric conditions the trum- 

 pets proved superior— the "selective power" of the atmosphere being 

 more noticeable in autumn than in summer. The results, however, 

 were for a time, inconstant and unsatisfactory, and with hardly a gleam 

 of f)rinciple to connect them. 



" It is," says Prof. Tyndall, "as already shown, an opinion enter- 

 tained in high quarters, that the waves of sound are reflected at the 

 limiting surfiices of the minute particles which constitute haze and fog, 

 the alleged waste of sound in fog being thus explained. If, however, 

 this be an efficient practical cause of the stoppage of sound, and if 

 clear, calm air be, as alleged, the best vehicle, it would be impossible 

 to understand how, to-day, in a thick haze, the sound reached a dis- 

 tance of twelve and three-fourths miles, while on May 20, in a calm 

 and hazeless atmosphere, the maximum range was only from five to six 

 miles. Such facts foreshadow a revolution in our notions regarding 

 the action of haze and fogs upon sound." 



The explanation, however, was not long deferred. On July 3, it 

 was determined that the vapor generated at the surface of the sea by 

 the sun's rays, rising and breaking through the air at various points, 

 presented the conditions of limiting surfaces producing partial echoe.s 

 and a consequent waste of sound. 



