302 TERRESTRIAL MAGNETISM.—MURPHY. 
magnetizing force in the opposite direction, the change of magnet- 
ization will be very great. 
“5, Tron, nickel and cobalt all probably have a maximum of 
magnetization, although its existence can never be entirely 
established by experiment, and must always be a matter of 
inference. 
“6. The permeability of any metal depends on the quality of 
the metal, on the amount of permanent magnetization, on the 
total magnetization, and on the temperature. 
“7. The permeability of cobalt and nickel varies very much 
with temperature, etc. The permeability of iron is not much 
affected by moderate changes of temperature. 
“8. The maximum of magnetization of iron and nickel de- 
creases with rise of temperature at least between 10° C. and 220° 
C., the first slowly, the second very rapidly.” 
Professor Chrystal adds :—* The researches of Stoletow and 
Rowland have undoubtedly made clear the main phenomena of 
magnetic induction, but in doing so they have raised a host of 
other questions which have not as yet been settled. * * * 
The results of the different experiments are not seldom contra- 
dictory, and the circumstances of experiment are often so com- 
plicated that criticism with the view of reconciling them seems 
hopeless in the meantime.” 
Now, I quote these authorities to show that there exists a 
relation of magnetism to other physical properties of bodies that 
may materially affect by mechanical strain (such as jarring or 
vibration) the tension of iron or steel. The following parallel 
statements, taken from the results of Weidemann, who has de- 
voted much careful study to these ee set will sufficiently 
illustrate the matter :— 
“1. Jarring a body under 1. Jarring a bar under 
twisting stress causes an in- magnetizing force causes in- 
crease of twist. crease of magnetization. 
2. Permanent twist in a 2. Permanent magnetiza- 
wire is diminished by jarring. tion in a bar is diminished by 
jarring. 
