156 POPULAR LECTURES AND ADDRESSES. 



central mounting. Here again is another model 

 illustrating waves (Fig. 34). 1 The white circles on 



1 This apparatus, which is represented in the woodcut, Fig. 34, is 

 of the following dimensions and construction. The series of equal 

 and similar bars (B) of which the ends represent molecules of the 

 medium, and the pendulum bar (P), which performs the part of 

 exciter of vibrations, or of kinetic store of vibrational energy, are 

 pieces of wood each 50 centimetres long, 3 centimetres broad, and 

 I '5 centimetres thick. The suspending wire is steel pianoforte wire 

 No. 22 B. W. G. ( '07 of a cm. diameter), and the bars are secured 

 to it in the following manner. Three brass pins of about '4 of a 

 centimetre diameter are fitted loosely in each bar in the position 

 indicated ; i.e. forming the corners of an isosceles triangular figure, 

 with its base parallel to the line of the suspending wire, and about 

 I mm. to one side of it. The suspending wire, which is laid in 

 grooves cut in the pins, is passed under the upper pin, outside the 

 pin at the apex of the triangle, over the upper side of the lower pin, 

 and thence down to the next bar. The upper end of this wire is 

 secured by being taken through a hole in the supporting beam and 

 several turns of it put round a pin placed on one side of the hole, as 

 indicated in the diagram. To each end of the pendulum bar is made 

 fast a steel spiral spring, as shown ; the upper ends of these springs 

 being secured to short cords which pass up through holes in the 

 supporting beam, and are fastened by two or three turns taken 

 round the pins. These steel springs serve as potential stores of 

 vibrational energy alternating in each vibration with the kinetic 

 store constituted by the pendulum bar. The ends of the vibrating 

 bars (B) are loaded with masses of lead attached to them. The 

 much larger masses of lead seen on the pendulum bar, which are 

 adjustable to different positions on the bar, are, in the diagram, 

 shown at the smallest distance apart. The lowermost bar carries 

 two vanes of tin projecting downwards, which dip into viscous 





