188 Sir William Thomson [Feb. 2, 



hofer with the letter A, has for wave-length 7J-100,000th of a centi- 

 metre. On the model before you I will now show you what is meant 

 by a " wave-length ; " it is not length along the crest, such as we 

 sometimes see well marked in a breaking wave of the sea, on a long 

 straight beach ; it is distance from crest to crest of the waves. [This 

 was illustrated by a large number of horizontal rods of wood con- 

 nected together and suspended bifilarly by two threads in the centre 

 hancrinc from the ceiling: : * on moving the lowermost rod, a wave 

 was propagated up the series.] Imagine the ends of those rods to 

 represent particles. The rods themselves let us suppose to be in- 

 visible, and merely their ends visible, to represent the particles acting 

 upon one another mutually with elastic force, as if of indiarubber 

 bands, or steel spiral sinkings, or jelly, or elastic material of some 

 kind. They do act on one another in this model through the central 

 mounting. Here again is another model illustrating waves (Fig. 2).t 

 The white circles on the wooden rods represent pieces of matter — I 

 will not say molecules at present, though we shall deal with them as 

 molecules afterwards. Light consists of vibrations transverse to the 

 line of propagation, just as in the models before you. 



* The details of this bifilar suspension need not be minutely described, as the 

 new form, with a single steel pianoforte wire to give the required mutual forces, 

 described below and represented in Fig. 2, is better and more easily made. 



t This apparatus, wliich is represented in the woodcut. Fig. 2, is of the follow- 

 ing dimensions and description. The series of equal and similar bars (B) of 

 wilich 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 1 • 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 as indicated ; i.e. forming the corners of an isosceles triangular 

 figure, with its base parallel to the line of the suspending wire, and about 1 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 liquid (treacle diluted with water) 

 contained in the vessel (c). A heavy weight resting on the bottom of this vessel, 

 and connected to the lower end of the suspending wire by a stretched indiarubber 

 band, serves to keep the lower end of the apparatus in position. The period of 

 vibration of the pendulum bar is adjustable to any desired magnitude by shifting 

 in or out the attached weights, or by tightening or relaxing the cords which pull 

 the upper ends of the spiral springs. 



