flocks and Regional Magnetic Disturbances. 523 



merit gave about 60 per cent. It is, of course, probable that if the 

 temperature is raised sufficiently the permeability of magnetite, 

 like that of iron, will, rapidly diminish, and that, after a certain 

 temperature is reached, it will cease to be magnetic. This point also 

 we hope to investigate further. 



PART II. On Regional Magnetic Disturbances. 



In attempting to base calculations upon the permeabilities measured 

 by Messrs. Highfield and Jarratt, it is necessary to make some assump- 

 tions as to the magnetic state of the earth's crust. 



The average increase of temperature with depth is about 1 C. for 

 every 90 feet, and if this rate obtains for a depth of several thousand 

 feet, the temperature would be 700 C. at 12 miles, or about 20 kilos, 

 from the surface. Iron ceases to be magnetic between 700 C. and 

 800 C., and it seems, therefore, fair to assume that below this depth 

 magnetic matter does not exist. Whether this be so or not, it is 

 necessary to suppose that at some given distance from the surface the 

 earth may be regarded as magnetically uniform. In selecting such 

 a distance for purposes of calculation the relations between the 

 magnetic properties of iron and temperature afford perhaps the most 

 trustworthy guide. 



Let then a level surface be regarded as homogeneous. It may be 

 called the magnetic floor. Let matter, magnetised by the earth's in- 

 duction, be supposed to be placed upon it, and let all calculations be 

 based on the hypothesis that the permeabilities with which we have 

 to deal differ but little from unity. 



The disturbance produced by the magnetic mass will be the same 

 whether the magnetic floor is magnetic or non-magnetic ; for the 

 same coating of south hemisphere magnetism, which in the latter 

 case will represent the effects of the earth's induction on the lower 

 surface of the mass, will represent the modification it produces in 

 the magnetisation of the floor if the latter is magnetic. 



The disturbances which have to be explained are of two kinds, viz., 

 those in which a very great range of vertical force disturbance 

 occurs within a limited area, and those in which a moderately high 

 value of vertical disturbance occurs over a large area. 



In all cases the effects of the upper and lower surfaces of the dis- 

 turbing masses will be opposed, and the force observed will be the 

 same if the disturbance is produced by similar masses, the distances 

 of which from the attracting point are proportional to their linear 

 dimensions. Even supposing, therefore, the shape and tho magnetic 

 properties of the mass are known, we learn nothing as to its prox- 

 imity to the surface from the mere magnitude of the disturbing force 

 at a single station. 



