Straining of the Earth resulting from Secular Cooling, 133 



the rotating plato has over the ordinary eye-piece micrometer, 

 is that its constant remains the same for all distances of the 

 scale from the telescope*. 



The above form of parallel-plate micrometer may also be 

 advantageously substituted for the eye-piece micrometer in 

 the first instrument described. In this case it should be 

 placed between the objective and the reflecting mirror A, or 

 else mounted on the mirror-frame itself so as to rotate with it. 

 The last position enables the same instrument to be used 

 either in the method of comparison or in the method of coin- 

 cidence, but is objectionable both on account of the increased 

 weight of the moving parts and because of the liability of dis- 

 turbing the adjustment of the axis or mirror while manipulating 

 the micrometer. It is therefore better to place the rotating 

 plate in the first position indicated and adapt it to either 

 method of use if desired, by making it cover only one-half 

 the field of the telescope, that half of course which is 

 opposite the unsilvered half of the mirror when the coinci- 

 dence method is used, and opposite the silvered half when the 

 comparison method is employed. 



In closing I wish to express my thanks to Messrs. Francis 

 and Kathan, the mechanicians of the laboratory, for the care 

 exercised in the mechanical execution of these designs. 



University of Chicago, September, 1895. 



XVII. On the Straining of the Earth resulting from Secular 

 Cooling. By Charles Davison, M.A., F.G.S., Mathema- 

 tical Master at King Edward's High School, Birmingham f. 



ESTIMATES of the depth of the surface of no strain have 

 hitherto been founded on the assumptions that the con- 

 ductivity and the coefficient of dilatation are constant J. In 

 the present paper, I propose to calculate the depth on the 



* The great practical advantage of this form of micrometer over the 

 ordinary form is its much greater simplicity and cheapness. For 

 these reasons it would have been adopted in all of the above instruments, 

 had it not happened that we already had on hand a number of micrometer 

 eye-pieces which were available for this purpose. 



t Communicated by the Author. A paper with the same title was 

 read before the Koyal Society on Feb. 15, 1894. The present paper 

 contains the substance of the former, but has been rewritten. 



X Phil. Trans. 1887 A, pp. 231-249; Phil. Mag. vol. xxv. 1888, 

 pp. 7-20. 



