126 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK 11. 



which does not produce a note depending upon its diameter and 

 distance from the head." The determining elements in a bell's pitch 

 may be generally represented by the "compromise" note of unison 

 rods A, A, starting from the head, and constrained by the ring effect 

 of the circumference. 



The illustrious encyclopedist also makes reservations on the rules 

 by which it is supposed that the tone of a bell can be determined 

 by its form and weight : he states that " it would be necessary to 

 calculate the elasticity and cohesion of the portions of the materials 

 of which they are cast, two elements on which only vague conjectures 

 can be formed." Experiment and the ear determine it with most 

 certainty. 



Flu. 79. Section of a bell. 



Flu. 80. Outside vie. v. 



Contemporary physicists admit that, other things being equal, the 

 dimensions of masses of similar form and material are in the inverse 

 ratio of the corresponding dimensions. It is by means of this law 

 that a series of bells of different dimensions, giving out the suc- 

 cessive notes. of the gamut and their modulations, can be produced. 

 Figure 82 represents an ancient instrument of this kind called 

 sonnantes, the bells of which, struck by two rods, were fixed on a box: 

 which strengthened the tone. It was a kind of harmonica with 



o 



metal bells. Franklin's harmonica consists of a series of glass bells, 

 or simply glasses with feet, which are put into vibration by fric- 

 tion ; this is done by the fingers or a ribbon being wetted, and 

 rubbed against the edges of the vessels. By pouring more or less 



