76 president's address — section c. 



be burst by the bismuth as the latter passes into the solid 

 state. 



Cracks therefore may be jiroduced in the crust (a) into 

 which molten material may be squeezed, derived either from 

 the re-fusion, consequent on the relief of pressure, of portion 

 of the already solidified zone (b), or from the Hquid or 

 potentially liquid nucleus of the earth below the latter zone. 



It may be objected at this point that the opposing forces 

 of the expanding- zone (b) and the contracting zone (a) may 

 so exactly balance one another that no squeeze will be 

 exerted on the nucleus, a.nd there will therefore be a zone of 

 no strain between the exterior of the nucleus and the lower 

 limit of zone (b). This might conceivably be the case if 

 the radial and circumferential expansion of zone (b) pro- 

 ceeded at an equal rate, so that the surface of zone (b) would 

 not undergo any appreciable deformation, and provided zone (b) 

 possessed the necessary rigidity to not only sustain its own 

 weight without support from the underlying nucleus, but also 

 to bear the weight and contractile force of zone (a), — an 

 exceedingly improbable hypothesis, which might be illustrated 

 by the following example, which ought to be specially familiar 

 to Australians. If the axle of a vehicle becomes jammed in 

 the axle-box owing to the axle becoming overheated, the 

 axle-box can be loosened either by chilhng the axle or heating 

 the axle-box. If the latter course be adopted, the axle-box, 

 undergoing equal radial and circumferential expansion conse- 

 quent on being heated, gradually lifts itself away radially 

 from the axle, and so loosens its hold and ceases to compress 

 the axle ; and this expansion of the axle-box by heat will 

 ensue in spite of the slight compressory force which may be 

 exerted on its exterior by the wooden naves of the wheel. 

 In this example the wooden nave would represent zone (a) 

 the axle-box zone (b), and the axle the nucleus of the earth. 



It is, however, improbable that the solidifying zone would 

 possess such an enormous strength and rigidity as this 

 hypothesis would demand, and consequently a squeezing 

 force would be exerted on the nucleus, which would tend to 

 force up its material into any cracks formed in zone (a) or 

 zone (b), and so produce volcanic outbursts. 



In the third stage of cooling it may be assumed that a 

 third zone (c) undergoes solidification, while (b) passes into 

 the contractile position. Zone (a), however, having lost 

 nearly all its heat capable of rapid radiation, radiates heat so 

 slowly that its contraction practically ceases. As, however, 



