WILLIAM SIEMENS, R&S. 301 



which would in fact urge the shot along. It was important to 

 reduce the inertia of the elastic medium to the lowest possible 

 amount, in order that it might instantly obey a change of pressure. 

 The motion of the centre of the diaphragm was imparted to a 

 scribing point in contact with a disc, made to rotate, during the 

 flight, with a given velocity. It appeared difficult at first sight to 

 obtain a uniform velocity of this disc without clockwork, which 

 was evidently inadmissible ; but an arrangement had occurred to 

 him, by which he expected to effect that purpose. He fixed upon 

 the disc two small fuses, or rockets, acting in opposite directions. 

 If both these rockets were made of equal power, it was evident 

 that no rotating motion would ensue ; but the one being made 

 equal to only about two-thirds of the other, the more powerful jet 

 would accelerate the wheel, until it was balanced by the lesser jet, 

 on account of the negative motion imparted to it. A moderate 

 and remarkably uniform rotation might thus be produced, for the 

 power of the larger jet would diminish, as the square of the 

 diminished relative velocity between the escaping gases and the 

 wheel ; whereas the power of this counter jet would increase, as 

 the square of the increased relative velocity between the gases and 

 the wheel. A small retardation of the wheel, by friction, or other- 

 wise, would consequently produce a great change in the relative 

 power of the two jets. These fuses were lighted the instant the 

 shot was dropped into the gun. Cards of zinc plate were fixed to 

 the sides of the rotating disc, covered first with a black and then 

 with a white varnish, whereon the scribing point would trace a 

 very clear line. Whilst this wheel revolved, a circular line would 

 be obtained, until the pressure upon the steel disc caused the 

 scribing point to ascend, producing a spiral indication of the pressure 

 at all intervals of time. The disc in the front of the projectile 

 was much lighter, being intended to indicate the resisting pressure 

 of the atmosphere, by a line upon the other side of the rotating 

 wheel. The negative pressure of the atmosphere against the back 

 of the projectile might, also, be recorded by a similar arrangement. 

 The diaphragm behind, should, in that case, be made very slight, 

 and be covered by a strong metallic plate, to resist the force of the 

 gunpowder, which plate would separate from the projectile at the 

 mouth of the gun. In the same way, the pressure upon any por- 

 tion of the curvilinear front surface of the project ile might bo 



