544 THE POPULAR SCIENCE MONTHLY. 



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

 rotating disk are employed, but radial slits are used instead of cir- 

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

 conical trumpet 16^ 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 rotating one, which is driven by separate mechanism. The trum- 

 pet is mounted on a boiler. In our experiments, steam of TO 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 depending on the velocity 

 of rotation. 



To the siren, trumpets, and whistles, were added three guns an 

 18-pounder, a 5^-inch howitzer, and a 13-inch mortar. In our summer 

 experiments all three were fired ; but the howitzer having shown itself 

 superior to the other guns, it was chosen in our autumn experiments, 

 as not only a fair but a favorable representative of this form of signal. 

 The charges fired were for the most part those now employed at Holy- 

 head, Lundy Island, and the Kish light-vessel namely, 3 lbs. of pow- 

 der. Gongs and bells were not included in this inquiry, because pre- 

 vious observations had clearly proved their inferiority to the trumpets 

 and whistles. 



On the 19th of May the instruments tested were : 



On the top of the cliff: 



1. Two brass trumpets or horns, 11 feet 2 inches long, 2 inches in 

 diameter at the mouthpiece, and opening out at the other end to a dia- 

 meter of 22^ inches. They were provided with vibrating steel reeds 

 9 inches long, 2 inches wide, and inch thick, and were sounded 

 by air of 18 lbs. pressure. 



2. A whistle shaped like that of a locomotive, 6 inches in diameter, 

 also sounded by air of 18 lbs. pressure. 



3. A steam- whistle, 12 inches in diameter, attached to a boiler, and 

 sounded by steam of 64 lbs. pressure. 



At the bottom of the cliff: 



4. Two trumpets or horns, of the same size and arrangement as 

 those above, and sounded by air of the same pressure. They were 

 mounted vertically on the reservoir of compressed air ; but within 

 about two feet of their extremities they were bent at a right angle, 

 so as to present their mouths to the sea. 



5. A 6-inch air-whistle, similar to the one above, and sounded by 

 the same means. 



The upper instruments were 235 feet above high-water mark, the 

 lower ones 40 feet. A vertical distance of 195 feet, therefore, sepa- 

 rated the instruments. A shaft, provided with a series of twelve lad- 

 ders, led from the one to the other. 



