118 ANNUAL OF SCIENTIFIC DISCOVERY. 



taken to fall. The pendulum, before an observation, is held suspended 

 at the left extremity of the arc, by means of a piece of soft iron in the 

 center of the bob, which is magnetized by an electric current through 

 an electro-magnet at the point of support. Connected with this mag- 

 net are two insulated wires, which pass away for 200 or 300 yards, 

 and terminate by the ends being wound across an upright screen, 

 30 feet in front of the gun, where the ends are joined, so that the 

 electric current is complete. Another instrument is employed, called 

 the conjunctor, and performs the following office: At the top of it is an 

 electro-magnet, which is connected by wires with the second screen, 

 120 feet in front of the first, i. e. 150 feet from the gun. These wires 

 are insulated with gutta-percha, &c., so that they may be either buried 

 in the ground or hung on posts. The electro-magnet in the con- 

 junctor retains a small weight suspended over a cup of mercury. This 

 has a steel blade above it, with a pin so arranged that if the weight 

 falls it presses it into the mercury. The pendulum has an index or 

 duplicate pendulum in rear, which is so attached to it by a light 

 spring that it will i'all and oscillate with the pendulum proper. Be- 

 hind the machine is a large electro-magnet, which has power to attract 

 this index when magnetized. 



Upon the gun being tired, the projectile cuts the wires of the first 

 screen, and thus demagnetizes the electro-magnet which holds up the 

 pendulum. This latter commences to fall, and the index needle with 

 it, along the graduated arc. When the second screen is cut through 

 by the shot, the electro-magnet at the top of the conjunctor is demag- 

 netized, and the weight falls into the mercury, pressing down the steel 

 blade, u;i,l completes another electric circuit, which magnetizes the 

 large magnet in the rear of the pendulum, and clamps the index needle 

 against the scale on the arc. The operator then reads off the scale 

 the distance which is marked by the index, and the time thus measured 

 is that which the shot took to pass through the screens, minus the time 

 necessary for the weight to fall in the conjunctor, and which the opera- 

 tor, before commencing, finds by means of the disjunctor. A table 

 has been prepared which shows the time for the pendulum to fall down 

 any arc fro u to 150, and only needs to be referred to for any arc 

 through which it falls when used as above. The time thus given has 

 only to be divided into the distance between the screens, and the result 

 will be the velocity of the projectile. 13y this contrivance a skillful 

 operator is able to measure pretty accurately to the y^Vo part of a 

 second. Sach wonderful precision renders this instrument most val- 

 uable for artillery scientific purposes ; and many most important 

 problems have been solved by its use. 



PRODUCTION OF SOUND BY ELECTRICITY. 



The following communication from Prof. Thomson, the well known 

 English scientist, is published in the London Chemical Gazette: 



Yesterday evening, when engaged in measuring the electrostitic capaci- 

 ties of som-) specimens of insulated wire designed for submarine tele- 

 graph cables I had occasion frequently to discharge through a g ilvanometer 

 coil, a comL-nser consisting of two parallel plates of metal, separated by 

 a space of a'r about -007 inch across, and charged to a difference of poten- 

 tials equal t-j that of about 800 Danicll's elements. I remarked at an in- 



