NATURAL PHILOSOPHY. 163 



ing its motion embarrassed ? Second, How, in a legible manner, 

 can the number of these rapid impulses be registered ? And, 

 third, By what means can the time of vibration be accurately 

 measured ? To register the vibration of cords and piano-wires, 

 the following arrangement of apparatus has been made. Through 

 the middle points of the vibrating cord passes a firm cambric 

 needle, the point of which will, when the cord is at rest, be very 

 near, but not in contact with the surface of mercury contained in 

 a cup beneath. A galvanic battery is connected, one pole with 

 this cup, the other with a trough, containing mercury, into which 

 dips the end of a wire bent twice at right angles, and turning 

 freely upon a hinge. To this wire is joined one end of the helix 

 of an electro-magnet, while to the other end of the coil is attached 

 a flexible metallic thread (a ravelHng of gilt lace) tied into the 

 eye of the needle, which passes through the vibrating cord. 

 Now, by the vibration of the cord, the needle-point will be brought 

 in contact with the surface of the mercury under it at the end of 

 every double vibration, and a current of electricity darts through 

 the wires, magnetizes the electro-magnet, which pulls the arma- 

 ture to its poles, and brings the registering point in contact with 

 the paper. As the paper is drawn swiftl}' along by clock-work, 

 while the armature with its sharp and soft lead-pencil point is 

 in motion, the vibrations of the cord are registered upon the jDaper 

 in a line of distinct black dots, easily counted. To measure time, 

 in the present form of the apparatus, a pendulum is used. The 

 pendulum when drawn to one end of its arc rests against one 

 arm of a lever, while the other arm carries a pair of pluckers 

 which grasps the cord. A pressure of the finger causes this finger 

 to release, at the same instant, the pendulum from one end, and 

 the cord from the other. The wire, whose lower end dips into the 

 trough of mercury, can at any time be brought into the arc of the 

 pendulum, by moving the block to which it is fastened, without 

 breaking the circuit ; but when this is done, the ball will strike its 

 upper end, and, knocking it over, lift the lower end from the 

 mercury, and open the circuit. The motion of the armature be- 

 gins with the beginning of the first vibration of the pendulum, 

 and stops at the end of 1, 3, or any odd number of seconds, and 

 the number of dots left upon the paper shows the number of 

 vibrations of the cord. The vibrations of the wire are made to 

 occur in a vertical plane opening and closing an electric circuit 

 with corresponding rapidity, and a dot is made upon the moving 

 paper at the moment when the lowest point in the vibration is 

 reached. The experiment begins when the wire is above its line 

 of rest ; the first dot, therefore, represents one-half of one com- 

 plete vibration. Should the experiment end when the wire is at 

 the highest point in its motion, the number of dots would show 

 the exact number of complete Aibrations made ; but since it may 

 end when the wire is at any point of its path, there may be a pos- 

 sible error of less than one-half of a complete vibration in an ex- 

 periment one second in length. As the time is lengthened, the 

 error is diminished ; in a registry of 5 seconds the maximum 

 error would be less than one-tenth of one vibration per second. 



