2 Rev. 0. Fisher on Variations of Gravity and their 



of the crust for the mountains themselves, and supposed an 

 attenuation equal to their mass to affect the crust for a depth 

 of 50 miles beneath them *. 



Geologists, however, would hardly be satisfied with this 

 explanation. For, although there are some areas which 

 appear to have been vertically lifted within comparatively 

 recent times, such as notably the Colorado Plateau, neverthe- 

 less the usual type of an elevated region is that of rocks 

 which have been heaped together by lateral pressure, so that 

 the material of which the mountains consist has been pushed 

 horizontally towards the range over the nucleus ; and this is 

 the case with the Himalayas. I have shown, in my c Physics 

 of the Earth's Crust/ that, if the substratum is plastic, this 

 process would involve the production of the kind of downward 

 protuberance that Airy postulates, and would explain the 

 existence of a deficiency of the attraction towards the moun- 

 tain-range. Accordingly, in the whole of my reasoning upon 

 the subject, needing some working hypothesis, I have assumed 

 that the crust is of the mean density of granitic rocks, viz. 

 2*68 ; and the substratum of the density of basic rocks, viz. 

 2*96. But I did not, in my book, make any quantitative 

 estimate respecting the probable result of my assumptions 

 upon the variation of gravity. 



My attention was again drawn to this question by reading 

 the instructive lecture delivered by General Walker, the 

 Superintendent of the Trigonometrical Survey of India, at 

 the meeting of the British Association at Aberdeen in 1885 f. 

 This led me to look into the description of the pendulum ex- 

 periments referred to in that lecture, which are published in 

 vol. v. of the ' Account of the Operations of the Great Trigo- 

 nometrical Survey of India ' J. If I understand the matter 

 rightly, what was done was in principle this : — At certain 

 stations of the Survey, of which the height and position had 

 been already determined, the mean number of swings, called 

 the "vibration-number/' was observed, which were made by 

 two pendulums in twenty-four hours which at the equator 

 would have made about 86,000 § vibrations in the same in- 

 terval. (It will be remembered that the number of seconds 

 in twenty-four hours is 86,400.) In this manner the force of 

 gravity at each station could be compared. The effect of 

 local attraction at the station was then estimated, as well as 



* f Figure of the Earth,' 4th ed. pp. 201 and 208. 

 t < Nature,' vol. xxii. p. 481 (1885). 



% Prepared under the directions of Major- General J. T. Walker, C.B., 

 R.E., F.B.S. Calcutta, 1879. 

 § Ibid. p. [129]. 



