374 THE VIBRATIONS OF HODS. 



A tuning-fork is a bent rod in which the two nodes 

 are very near to one another. Its mode of vibrating i 

 shown in fig. 225, from which it will be seen 

 that the handle of the fork does not coincide 

 with either of the nodes, but is between them, 

 so that when the fork vibrates, it is moved 

 vigorously up and down and produces a louc 

 note when pressed upon a sounding-board tc 

 which it can communicate its motion. In th< 

 I figure the extent of the motion of the forkHf I 



ffor the sake of clearness, much exaggerated. 

 Vibrating rods which swing freely produce only ver 

 FIG 225 faint notes, on account of their small surface, unles 

 Qrealsize). they are rather broad as in the glass-harmouicoti, whie 

 consists of strips of glass affixed to cords at the nods 

 points. To render the tone of a glass tube or steel rod distinct! 

 audible, a thread, from 30 to 50 cm long, is tied to it near on 

 end. The other end of the thread is wound round the tip of tli 

 forefinger and pressed by the latter into the ear. The freely hangin 

 rod is then sounded. 



Besides the transversal vibrations which have bee 

 described, rods are capable of performing longitudinj 

 vibrations, in which the particles vibrate in the directio 

 of the length of the rod. The vibrations are produce 

 by holding a rod in the middle and rubbing one half < 

 it, lengthwise, with the moistened finger or with a dam 

 cloth if the rod is of glass ; if it is of steel, it shoul 

 be rubbed with the finger over which some powderej- 

 rosin has been sprinkled. 



The notes produced by longitudinal vibration- 

 the higher, the shorter the rods, but are independei 

 of their thickness. The notes produced by transv 

 vibrations are the higher, the shorter and thicker tl 

 rods, and are in general much deeper than those arisin 

 from longitudinal vibrations. 



