CHANGES IN LENGTH OF DAY— BROWN ]^75 



matter inside the earth, different densities playing the principal role 

 in this hypothesis. I repeat, however, that these are mere hypo- 

 theses made to account for the phenomenon. 



My own idea is to imagine the existence of a layer of material not 

 too far from the surface which is at or near a critical temperature, the 

 latter being defined as one in which a small change of temperature 

 produces a relatively large change of volume. There are many sub- 

 stances which possess this property. Thus a small change in the 

 interior condition of the earth might easily produce a relatively large 

 change in the volume of the layer, causing necessary expansion or 

 contraction at the only portion which is free to move, namely, that 

 above the layer. 



A change of volume considerably less than 1 percent in a layer with 

 a thickness of a mile would be amply sufficient to take care of the 

 maximum observed change. Incidentally, it may be noted that the 

 change in volume consequent on any change in temperature gives a 

 rapid method of transfer of heat in a mass composed of different ma- 

 terials, for changes of pressure are transferred without delay and the 

 cooling of a hot mass could be made much more rapid in this way than 

 by mere conduction. 



The hypothesis gives a mechanism for mountain building which 

 has some rather attractive features. The elasticity of the surface 

 materials of the earth is amply sufficient to take care of any such 

 increase in the volume of its crust. As a matter of fact, however, the 

 surface is broken and fissured in all du'ections, and the effect of an 

 expansion would be to open these fissures. They would be partially 

 filled by matter dropping from above or i)ressed in from below. In 

 any case, when the subsequent contraction came, they would not be 

 able to close and there would necessarily be a bulging toward the 

 surface. The evidence we have indicates that the major changes take 

 place at intervals of the order of a century or less, so that the same 

 fissures are likely to repeat their successive openings and closings, 

 resulting in successive elevations of the same region. The energy 

 necessary for the process is thus traced back to the interior heat of 

 the earth and the supply would seem to be ample. 



It is not entirely impossible that the hypothesis could be tested if 

 and when the next great change occurs. Some device which could 

 measure very small changes in the opening of a fissure could certainly 

 be set up. If one costing a small sum only could be constructed and 

 numbers of them placed in various parts of the earth, especially in 

 those regions where mountain building is known to be going on, the 

 information needed could be obtained. In a few places elaborate 

 mechanisms with recording devices might be employed. In any case, 

 much might be learned concerning the movements which are continu- 

 ally taking place on the surface of the earth. 



