372 
ilil. C. CHREE ON THE EFFECTS OP PRESSURE 
large extent independent of the precise character of tlie change of stress, and would 
doubtless follow more or less any mechanical agitation of the rod. 
The curve h is drawn through nearly all the experimental points, but in the stronger 
fields of c and d, especially the latter, the experimental points are somewhat widely 
scattered. These latter curves accordingly cannot be trusted in their minute details. 
The curves are based on the data in the fourth columns of Tables VI., VII., and VIII. 
These tables give only certain average results for fields over 90 C.G.S. units, but they 
indicate that the ordinates of h and c would attain maxima values in fields someAvhere 
between 100 and 200 C.G.S. units. Thereafter the ordinates of c would apparently 
diminish the more rapidly of the two, so that in fields of 400 C.G.S. units the curve c 
woidd have approached pretty close to the curve d, which remains apparently nearly 
horizontal. 
§ 53. The application of cycles of pressure causes cyclic changes in the magnetisa¬ 
tion, whose character will presently be discussed. In consequence of this the effects 
of the first change of stress on the residual magnetisation are only, in part, of a non- 
cyclic character. To get the exact quantity of magnetisation whose •permanent 
disappearance is secured by the first change of stress, the cyclic effect must be 
allowed for. 
Suppose, for instance, a field of 50 C.G.S. units is broken when pressure is “off” 
The cyclic effect is then a diminution of magnetisation when pressure is “ on.” 
Consequently the loss of magnetisation accompanying the first application of pressure 
exceeds the quantity whose permanent disappearance is secured by the amount which 
takes part in the cyclic change. Subtracting from the total loss the numerical value 
of the cyclic effect, we get what may be regarded as the true loss of residual magneti¬ 
sation due to the first application of pressure. This quantity is that here termed the 
shock-effect of the first pressure. 
In the case of residual magnetism the cyclic effect is in general small compared to 
the shock-effect, so that a considerable error in the cyclic effect will seldom seriously 
affect the calculation of the shock-effect. 
§ 54. In fig. 15 the ordinates represent the ratio of the shock-effect of the first 
alteration of pressure to the amount of the pre-existing residual magnetisation. The}^ 
may thus be regarded as measuring the efficacy of the first change of pressure in 
shaking out the residual magnetisation. Unfortunately no readings were taken of 
the cyclic effects of pressure in the weaker fields in the series of experiments on which 
the curves are based. Thus up to the points where the transverse lines are drawn, 
the ordinates represent the ratio of the total effect of the first change of pressure to 
the initial residual magnetisation. Gonsequently, as will be seen presently, in the 
commencing j^ortions of the curves the ordinates of 6 and c are probably somewhat 
underestimated, and those of d, on the other hand overestimated. The corrections 
could, however, hardly modify the relative positions of the curves. 
The curves all show that the efficacy of the first change of pressure in shaking out 
