562 Dr. J. H. Vincent on the Construction of a Mechanical 



The unbroken vertical lines represent the values of b and a. 

 The free frequency of the suspended weights (which are a 

 series of simple pendulums) is '66 per sec. The value of a 

 is obtained by multiplying this number by the ratio of tbe 

 sum of the masses of bullet and hanger to that of the bullet. 



As the frequency increases from to b, the wave-length 

 and velocity decrease. The lower system moves in the same 

 phase as the upper system in this part of the curve. When 

 n=b the lower particles are thrown into violent agitation ; 

 but when « is a little greater than b the model refuses to 

 propagate the motion at all. The effect is most striking 

 when n approaches a. In this case the model presents a 

 most curious appearance ; the metronome may be left driving 

 the model for a long while, but the only effect is a slight 

 bending near the point of attachment to the driving apparatus. 

 The whole model comports itself as if made of some pliable 

 non-elastic material. 



When n is a little greater than a, the waves can again be 

 observed. As n increases, the wave-length and velocity 

 decrease. The upper and lower system move in opposite 

 phase in this portion of the curve near a ; when the frequency 

 is higher the hanging weights are unaffected, and the velocity 

 becomes independent of the frequency. 



Comparison of Theoretical and Experimental Curves. 

 The former has been drawn so as to coincide with the 

 latter when n = 2. Although the two curves are in general 

 alike, a marked discrepancy occurs when the frequency is 

 low. This is due to the controlling influence of the threads 

 by which the bullets are hung from the roof. One would 

 expect the effect to become noticeable when the frequency 

 of the waves traversing the model begins to approach the 

 value of the free frequency of a particle suspended from the 

 roof at the same height as tbe bullets. This frequency is 

 *3 per second, and is shown in fig. 2 as a broken line. The 

 effect is in the direction to be expected, as an extra force of 

 restitution would increase the velocity of propagation. 



The Apparatus as an x-Ray Model, 



Soon after the discovery of a?-rays, Sir George Stokes 

 suggested that they consisted not of periodic vibrations but 

 of pulses. Prof. J. J. Thomson has lately given a mathe- 

 matical theory, according to which the Rontgen rays consist 

 of very thin electromagnetic pulses of great intensity, due to 

 the sudden stoppage of the cathode carriers. 



In this theory the thickness of the pulse may be regarded 

 as analogous to wave-length in optics. 



