220 Intelligence and Miscellaneous Articles. 



considerable error that has been introduced into the most impor- 

 tant measurement of this quantity. 



The experiments were made at the United-States Arsenal in 

 Watertown, Mass. 



The method used was an automatic measurement of the velocity 

 at different distances (varying from 10 to 110 feet) from the mouth 

 of the cannon, by means of a series of membranes* electrically 

 connected with a chronograph. 



In the midst of a large level field was placed a six-pound brass 

 field-piece. In the rear of this, at distances of 10, 30, 50, 70, 90, 

 and 110 feet from the mouth of the cannon, were placed the mem- 

 branes, elevated about 3 feet above the ground. These membranes 

 consisted each of a hoop 9 inches in diameter, over which was 

 stretched a sheet of thin rubber. To the centre of the membrane, 

 and on the side towards the cannon, was attached a very small 

 shelf of polished brass. Upon this rested one end of a delicate 

 steel spring, the other end being fixed to an independent support. 



The wire th^it brought the current of electricity from the chro- 

 nograph-house was connected with the spring ; and from the shelf 

 a second wire returned to the chronograph. When the spring 

 rested upon the shelf the circuit was closed ; the passage of the 

 sound-wave, however, would move the membrane, and break the 

 circuit, causing a register on the chronograph. When the spring 

 fell it rested upon a contact-point, from which a wire ran to the 

 next membrane of the series ; so that the circuit, immediately after 

 being broken at the first membrane, was made again through the 

 second before the sound-wave reached it. In this way the current 

 could be transferred to all the membranes of the series, and the 

 successive breakings and makings of contact, as the sound-wave 

 passed each one, could be registered on a chronograph placed at a 

 distance. 



The chronograph used was of the Schultz form, and consisted 

 essentially of a rapidly and uniformly revolving cylinder of silver, 

 covered with lampblack, which was made one pok of the secondary 

 coil of an inductorium, the primary coil of which was in circuit 

 with the membranes. The other pole of the secondary coil was a 

 fine metal point brought very near to the surface of the cylinder. 

 When the primary circuit was broken or completed at the mem- 

 branes, a spark passed between the metal point and the cylinder, 

 and made a fine dot in the lampblack. By the side of the point 

 was an electrical tuning-fork, which traced a sinuous curve of times 

 on the lampblacked surface of the cylinder. The time could thus 

 be measured to *00001 of a second. All that was necessary then 

 for the experiment was to choose a moment when the air was as 

 nearly as possible at rest, and then, the membranes being in order, 

 to start the chronograph and fire the gun. The distances between 

 the membranes were then accurately measured, the times of passage 

 between successive membranes determined from the chronograph, 



* Regnault used membranes, though unlike these, in his water-pipe 

 experiments. 



