298 Prof. A. Gray and Mr. A. Wood. Effect of a [May 1, 



Curves 1, 2, 3, 4 for fields up to 156 C.G.S., lie successively farther 

 to the left in the diagram, while the Curves 5, 6, 7, 8 for fields from 

 187 to 360 C.G.S. lie in succession to the right of one another, showing 

 in the first case a continual increase, and in the second a continual 

 diminution in the rate of subsidence. 



10 



§ 20 



I 



o 



30 



AO 



50 



Amplitude. 



30' 



60 ' 



90" 





























/// 









































5 L / 















Diagram II. 



Curve. 

 1 

 2 

 3 

 4 



Field. 

 C.G.S. 

 22 „ 

 50 „ 

 156 .. 



Curve. 



Field. 

 187 C.G.S. 



257 „ 

 330 „ 

 360 „ 



A curve for zero field was taken after these eight curves had been 

 obtained, and it was found to agree closely with Curve 1 of the series 

 shown in the diagram. 



The logarithmic decrements were calculated for these curves and 

 were found to diminish in each curve as the amplitude diminished. 

 This diminution was about half the initial value of the logarithmic 

 decrement for each of the first four curves, but then fell off until in 

 curve 8 it was only \ of the value at the initial amplitude. 



A series of experiments was made for the purpose of finding more 

 precisely the value of that magnetising field which had maximum 

 effect upon the rate of subsidence, and curves of subsidence were 

 obtained for values of the field ranging from about 100 to about 

 200 C.G.S. The curves form a close series, and intersect one another 

 in a way which it is rather difficult to disentangle, and which renders 

 the critical value of the magnetic field in a sense indefinite. The 

 crossing of the curves in the immediate neighbourhood of the critical 



