462 ANNUAL KECORD OF SCIENCE AND INDUSTRY. 



mile. It was arranged within a closed mirror in such a 

 manner that the beam of light was projected against the 

 clouds, which served as a screen. In front of the mirror the 

 signals were made, which were duplicated upon the clouds 

 on a gigantic scale. This process, it is said, is to be adopt- 

 ed by the German army for night-signaling. 6 Z>, XXXIII., 

 281. 



FOG-SIGNALING BY EXPLOSIVES. 



Major Maitland, of the Royal Gun Factory at Woolwich, 

 recently delivered a very fully illustrated lecture before the 

 Royal United Service Institution on fog-signaling by means 

 of explosives ; a lecture which was a continuation of a pre- 

 vious address b}'^ Vice-Admiral Collinson on the use of horns, 

 whistles, the siren, etc., as fog-signals. Major Maitland states 

 that three kinds of guns were experimented upon by the Trin- 

 ity Board with regard to their appropriateness as signal guns ; 

 and that body decided to apply to the War Office for a solu- 

 tion of the following problem : " To prod uce the most far-reach- 

 ing sound possible with a charge of three pounds of powder, 

 and to enable two men to keep up the firing at intervals of 

 not more than five minutes during a long -continued fog." 

 The problem divides itself into two parts : the ease of work- 

 ing and the propulsion of sound. With regard to the latter 

 and more difficult question, he states that the designing of a 

 gun is a simple problem, but the production of sound is quite 

 another matter. We have hitherto looked upon the noise of 

 a discharge as a necessary evil; now, however, we have to 

 investigate the cause of the report of the gun, and seek to 

 construct such an apparatus as will give the greatest noise 

 with the least possible expenditure of powder. The sound of 

 a discharge is made up of two factors ; one, the blow struck 

 by the expanding gas on the still air at the muzzle of the 

 gun ; the other, the vibration of the metal of the gun. It is 

 found that when a gun is fired, a great difference in the in- 

 tensity of the sound is experienced by the observer, accord- 

 ing to whether he places himself a little before or behind a 

 line drawn at rioht an2;les across the muzzle of the cfun. It 

 would appear that the rapidly expanding gas strikes a vio- 

 lent blow on the air in front of the muzzle, and, failing to 

 drive the mass of air at once, sends its vibrations forcibly 



