Prof. Tyndall on the Vibrations of Strings. 73 



the wire, which now glows brightly. I sound the fork : the wire 

 vibrates as a whole ; its two ends are brilliant, but its middle is 

 chilled by its rapid passage through the air. Thus you have a 

 shading off of incandescence from the ends to the centre of the wire. 

 I relax the tension : the wire now divides itself into two vibrating 

 parts ; I relax still further and obtain three ; still further, and now 

 you have the wire divided into four ventral segments separated from 

 each other by these three brilliant nodes. Right and left from every 

 node the incandescence shades away until it disappears. You 

 notice also that when the wire settles into steady vibration the nodes 

 shine out more brilliantly than the wire shone before the vibration 

 commenced. This is due to the cooling of the vibrating segments. 

 When they are chilled by their swift passage through the air, their 

 conductivity is improved. More electricity passes through the 

 vibrating than through the non-vibrating wire, and hence the aug- 

 mented glow of the nodes. 



31. Certain twisted cords, as I have said, when attached to 

 tuning-forks execute circular instead of plane vibrations. Circular 

 vibrations may always be obtained by attaching one end of a string 

 to a rotating point, and timing the velocity of rotation to suit the 

 periods of vibration possible to the string. 



32. Thus, suspended from the ceiling is this white cotton rope, 

 the lower end of which is attached to a hook belonging to this whirl- 

 ing table. When this handle is turned the hook rotates, describing 

 a circle less than an inch in diameter. By properly regulating the 

 velocity of rotation I cause the rope to divide into a series of spin- 

 dles, which appear like grey gauze when projected against a dark 

 background. I can cause the number of these spindles to vary from 

 one to twenty, by varying the velocity of rotation. 



33. In this experiment the rope is vertical and the rotating wheel 

 horizontal ; but 1 hate here an arrangement in which the wheel is 

 vertical and the vibrating chord horizontal. Instead of a rope I now 

 employ a burnished silver chain. Turning this wheel slowly, I cause 

 the chain to swing as a whole, and describe a spindle 12 feet long 

 and nearly 2 feet in diameter at its widest part. Augmenting the 

 speed of rotation I divide the chain into two spindles, each 6 feet 

 long and more than a foot in diameter ; a still higher velocity gives 

 us three spindles, a still higher four, and so on. The gas-light here 

 flashes at intervals from the burnished links of the chain, and each 

 of those brilliant points describes a curve of light. 



34. As soon as synchronism is established between the wheel and 

 the chain, the amount of work performed by the arm which turns 

 the wheel is very sensibly augmented. Indeed it may be augmented 

 until the strain thrown upon the chain by the accumulation of small 

 impulses is sufficient to break it. 



35. I substitute for the chain this lighter cotton rope, round which 

 a silver band has been coiled as a spiral ; and from an electric lamp 

 placed at the end of the rope I send a beam of light along its entire 

 length. I now turn the wheel and divide the rope successively into 

 two, three, four, or five ventral segments. The spindles show a gra- 

 duated brilliancy, which is intensified until it becomes almost daz- 



