1878.] Prof. Tyndall. Experiments on Fog-Signals. 249 



the open, and the gunpowder fired from the new gun, the mean values 

 of their sounds were found to be the same. Fired in the focus of the 

 reflector, the gun-cotton clearly dominated over all the other sound- 

 producers.* 



The whole of the observations here referred to were embraced by 

 an angle of about 70°, of which 50° lay on the one side and 20° on the 

 other side of the line of fire. The shots were heard by eleven observers 

 on board the " Gralatea," which took up positions varying from 2 miles 

 to 13 J miles from the firing-point. In all these observations, the rein- 

 forcing action of the reflector, and of the parabolic muzzle of the gun, 

 came into play. But the reinforcement of the sound in one direction 

 implies its withdrawal from some other direction, and accordingly it was 

 found that at a distance of 5J miles from the firing-point, and on a line 

 including nearly an angle of 90° with the line of fire, the gun-cotton 

 in the open beat the new gun ; while behind the station, at distances 

 of 8f miles and 13^ miles respectively, the gun-cotton in the open 

 beat both the gun and the gun-cotton in the reflector. This result is 

 rendered more important by the fact that the sound reached the 

 Mucking Light, a distance of 13§ miles, against a light wind which 

 was blowing at the time. 



Most, if not all, of our ordinary sound-producers send forth waves 

 which are not of uniform intensity throughout. A trumpet is loudest 

 in the direction of its axis. The same is true of a gun. A bell, with 

 its mouth pointed upwards or downwards, sends forth waves far 

 denser in the horizontal plane passing through the bell than at an 

 angular distance of 90° from that plane. The oldest bellhangers 

 must have been aware of the fact that the sides of the bell, and not 

 its mouth, emitted the strongest sound, their practice being deter- 

 mined by this knowledge. Our slabs of gun-cotton also emit waves 

 of different densities in different parts. It has occurred in the experi- 

 ments at Shoeburyness that when the broad side of a slab was turned 

 towards the suspending wire of a second slab six feet distant, the wire 

 was cut by the explosion, while when the edge of the slab was turned 

 to the wire this never occurred. To the circumstance that the broad- 

 sides of the slabs faced the sea is probably to be ascribed the remark- 

 able fact observed on the 23rd March, that in two directions, not far 

 removed from the line of fire, the gun-cotton detonated in the open 

 had a slight advantage over the new gun. 



Theoretic considerations rendered it probable that the shape and 

 size of the exploding mass would affect the constitution of the wave 

 of sound. I did not think large rectangular slabs the most favourable 

 shape, and accordingly proposed cutting a large slab iuto fragments 

 of different sizes, and pitting them against each other. The differences 



* In this case the reflector Tvas fractured hy the explosion, hut it did good service 

 after fracture. 



