SECULAR MOVEMENTS 225 



ions, and elect rons. which co-operate with gravity in accordance with laws of their 

 own. Their general effect is to make matter denser. The extent of their opera- 

 tion is undetermined, but there is ground for thinking that the density of the 

 interior still may be increasing by their action. It is known that substances which 

 i r\ -talli/e in a given way under surface pressures may be changed into denser 

 forms under higher pressures. Re-aggregation in the interior thus probably means 

 iin reased density, and it may be going on constantly. While knowledge on this 

 point is inconclusive, it is permissible to entertain the view that gravitational, 

 molecular, atomic, and sub-atomic forces have been and are still at work tending 

 to increase internal density. It is even conceived that this may be a chief (if not 

 the rhief) cause of earth-shrinkage. 



2. The resisting agencies. The condensing agencies are more or less held 

 in check by resisting agencies. Of these heat is the most familiar. It is abetted 

 by the molecular and atomic arrangements which exist at any given time, and 

 which resist change, and by factors in the ultimate structure of matter, not well 

 understood. It has been usual to regard the primitive state of the earth as one of 

 intense heat, and to assign its subsequent reduction of volume almost solely to 

 loss of heat; but this is not the view here favored. On the contrary, the heat of 

 the earth is supposed to have been developed chiefly by reduction of volume and by 

 radio activity, and the heat thus developed is one of the forces which check further 

 decrease of volume. Loss of heat is, of course, a cause of shrinkage, but its effect 

 is thought to be less than that of molecular and sub-molecular rearrangements of 

 the material of the earth, resulting in greater density. The loss indeed may not 

 be greater than the new heat generated in the shrinkage. 



Observed temperatures in deep excavations. As the earth is penetrated below ^. ( 

 the zone of seasonal changes, by wells, mines, tunnels, and other excavations, the t 

 temperature is almost invariably found to rise, but the rate of rise is far from 

 uniform. If we set aside as exceptional the unusually rapid rise near volcanoes 

 and in other localities of obvious igneous influence, the highest rates are more than 

 six times the lowest, the range being from 1 F. in 20 feet, to i in 135 feet, 1 with 

 an average of i in 50 to 60 feet. The recent deep borings in which temperatures , 

 have been carefully recorded, indicate a slower rate of rise, say i for 80 feet. It -^ 

 is not probable that the observed rates of increase continue to the center. One 

 degree in 60 feet, continued to the earth's center would give a temperature of 348,- 

 000 Fahr., and i Fahr. in 100 feet would give 209,000 Fahr. It is probable 

 that the rate of increase diminishes with depth, and that the temperatures cited 

 above are far in excess of those actually existing at the center. 



. 1 Huniiit of loss of heat and shrinkage. The amount of loss of interior heat may 

 be estimated from that which is observed to be passing outward through the rocks, 

 or by computations based on the estimated temperature gradients and with the 

 known conductivity of rock. Estimates of the loss of heat in 100,000,000 years 

 range from 10 C. (iS Fahr.) (Tait) to 45 C. (81 Fahr.), for the whole earth. 

 This is an exceedingly small result, and emphasizes the low conductivity of rock. 

 With this amount of cooling, the shrinkage resulting has been calculated. 

 For a loss of 10 C., the circumferential contraction is calculated to be 1.6 to 2.35 

 miles; for a loss of 45 C., 7.27 to 10.5 miles. These results are so small (cf.p. 223) 



1 i F. for 250' down to 8,000 feet, is reported from the Rand.,S. Af. Mining 

 World, Jan. 7, 1911, p. 2. 



