130 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



A NEW FOG-SIGNAL. 



Experiments made in England with gun-cotton in the 

 open air are said to have demonstrated that a mass of ten 

 ounces of compressed gun-cotton, fired by means of two 

 ounces of dry gun-cotton, as a primer, the whole being det- 

 onated with fulminate of mercury, produced a discharge 

 which could be heard very distinctly at a distance of ten 

 miles in all directions. These results were so satisfactory 

 that it has been determined to build a parabolic reflector 

 of cast iron, by which the intensity of the sound of the ex- 

 plosion of a charge of compressed cotton placed in its focus 

 will be greatly intensified in one direction. The trials of 

 the adaptability of this device as a fog-signal will be made 

 at the Royal Arsenal, Woolwich. The Engineer. 



NEW METHOD OF OBSERVING THE VIBRATIONS OF A TUNING- 

 FORK. 



A new method of determining: the absolute number of vi- 

 brations corresponding to any musical note is described by 

 Poske, and has a high value in comparison with those that 

 have hitherto been employed, which may be classified as 

 graphic, acoustic, and optic : the first of these three is compar- 

 atively rough ; the use of the siren is a good example of the 

 acoustic method, although its practicable employment is 

 found troublesome ; and of the optical, that of Lissajous 

 is in high esteem. The new method proposed by Poske 

 consists, first, in replacing the clock or chronometer by the 

 electro-magnetic rotation apparatus of Helmholtz, whose ve- 

 locity of rotation is extremely constant, and can be deter- 

 mined accurately to its ten-thousandth part. The essential 

 portion of this apparatus consists in a centrifugal regu- 

 lator, which diminishes the strength of the electric current 

 by the diminution of the number of contacts, as soon as the 

 velocity of rotation exceeds a certain limit. The observer 

 examines, through a microscope, a minute bright point upon 

 the vibrating rod or cord, which point by its vibrations ap- 

 pears as a bright line ; and between the eye and the vibrat- 

 ing point there also rotates a disk perforated with a known 

 number of slits. The combination of the revolving slits and 

 the vibrating point causes the latter to appear to move 



