PLANS FOR OBTAINING SUBTERRANEAN TEMPERATURES. 267 



pendulum, and the measurement of the earth's weight by means of 

 a vertical pair of gravity determinations could thus be repeated. 

 The homogeneity of the crust layer between the upper and lower 

 stations and the representative character of the rock samples brought 

 up as drill cores would be peculiarly favorable for the determination 

 of the density of the crust layer. 



To give high precision to the determination of density it would be 

 necessary to take account of the compression of the rock under stress 

 of the superincumbent weight. Rock compression has not yet been 

 measured in the laboratory, the matter being one of extreme diffi- 

 culty, by reason of the deformation of both samples and testing 

 apparatus when great pressures are applied ; but there is reason to 

 think that valuable observations bearing on this point could be made 

 within the boring at some stage of the work. It should be possible, 

 by suitable automatic appliances, to measure that resilient elongation 

 of the column of rock constituting a section of core which theoretic- 

 ally takes place while the drill is separating it from the general mass. 

 The importance to geophysics of experimental determinations of 

 rock compression is generally recognized. 



