NA rURE 



433 



THURSDAY, SEPTEMBER 



1S95 



THE PENDULUM AND GEOLOGY. 

 Rl'suIIs of a Transcontinental Series of Gravity Measure- 

 ments. Hy George Rockwell Putnam. Notes on the 

 Gravity Determinations Reported by Mr. G. R. Putnavi. 

 By drove Karl Gilbert. (Washington, U.S.A. : 

 Philosopliical Society's Bulletin, vol. xiii. pp. 31-76.) 



SINCE the ninnber of swings, which a pendulum of 

 given length makes in a certain number of hours, 

 depends upon the attraction of the earth at the place 

 ■where it is swinging, it follows that, if an observer carries 

 the same pendulum to different places, and notes the 

 number of swings at each place he visits, he can by that 

 means compare the force of gravity at the several places. 

 If the earth were a smooth spheroid consisting of con- 

 centric shells, each of uniform density throughout, then 

 gravity w-ould have the same value at all stations situated 

 on the same parallel of latitude. But if, as i? the case in 

 nature, there arc mountains and elevated plateaus along 

 the course followed by the observer, gravity ought to vary 

 from its normal value, and in fact it is found to do so. 

 Theoretically it is possible to calculate what variation of 

 gravity at a given station ought to be caused by the 

 altitude of the station, and the attraction of the neigh- 

 bouring visible masses — i.e. of the mountain or plateau 

 where the pendulum is swung, and of the rock masses 

 round about, and when these disturbing causes are 

 allowed for, and the corresponding corrections made, the 

 value of gravity as deduced from the rate of the pendulum 

 might be expected to tally with what it would be at the 

 base level, supposing the mountains and all the sur- 

 rounding masses carted clean away, and the smooth sur- 

 face of the globe laid bare. This correction is termed 

 reducing to the sea level, or to the mean level if the 

 reference is made, not to the sea, but to some inland 

 station. The question then to be answered for each 

 station is, whether when this correction has been made, 

 or, in technical language, when gravity has been reduced 

 to the sea, or mean, level, does the reduction give the 

 value which might be expected for the latitude? If it 

 does not, this points to some deviation from regularity in 

 the density of the earth's crust below the station, the 

 nature of which may be inferred from the character and 

 amount of residual discrepancy, when the reduction has 

 been made. In this w^ay it is that the pendulum becomes 

 a kind of geological stethoscope. 



In investigations of this kind, the elevated ground 

 which forms the station is usually very much wider than 

 it is high, so that, bearing in mind the law of the inverse 

 square, it may be regarded as an extensive plain. If 

 from local peculiarities it cannot be so regarded, com- 

 pensatory allowances are made to bring it under that 

 category. The effects of the station being situated on an 

 elevated plateau are of three kinds, two of which cause 

 gravity to appear smaller than it would appear at the sea 

 level beneath the station, and one which causes it to 

 appear greater. Of the two which make it appear 

 smaller, the more important is, that the increased distance 

 from the earth's centre causes the attraction of the earth 

 as a whole to be diminished ; the other, which is insig- 



NO. 1349, VOL. 52] 



nificant, and usually neglected, is that the increased dis- 

 tance from the axis of rotation increases the centrifugal 

 force, which is opposed to gravity. The third effect, 

 which causes gravity to appear greater than at the sea 

 level, arises from the attraction of the matter of which the 

 elevated plain, or mountain, is composed, for that maybe 

 regarded as an adventitious mass of rock, in excess of the 

 sphere, placed beneath the pendulum. The reduction of 

 the gra\ity observed at the station consists, therefore, in 

 adding a correction equivalent to the diminution due to 

 the elevation of the station, and subtracting a correction 

 equivalent to the attraction of the mass of the elevated 

 plain. If the reduction so made does not bring the ob- 

 served value to agree with the value at the sea level, 

 appropriate to the latitude of the station, there must be 

 some geological cause present to account for the 

 discrepancy. 



It came to light in 1847, in consequence of the great 

 trigonometrical survey of India, that, on approaching the 

 range of the Himalayas within about si.xty miles, the 

 plumb-line, or vertical, was slightly deflected towards the 

 mountains, so that it did not remain exactly perpen- 

 dicular to the earth's surface. This was what might have 

 been expected, because the great rocky mass would 

 naturally draw the plumb-line towards it. But when the 

 attraction of the mountains came to be calculated, it was 

 discovered that, although their action was great enough 

 to have caused a source of perplexity to the surveyors, it 

 was nevertheless not so great as might have been ex- 

 pected. Clearly, then, some geological cause was latent, 

 which required to be explained. 



After some not very successful attempts at explanation 

 by others, .'X.iry, then Astronomer Royal, proposed in 

 1855 a solution of the difficulty which met the case. He 

 assumed, as in those days was usually done, that the 

 crust of the earth was comparatively thin, and rested 

 upon a more or less liquid substratum, which in his 

 paper in the Philosophical Transactions he called " lava." 

 Then he showed that a great mountain mass would 

 break the crust through unless it was supported by a 

 protuberance beneath it, projecting downwards into a 

 layer denser than itself. In short, it needed to be held 

 up in hydrostatic equilibrium, much as an iceberg is 

 supported in the ocean ; and he explained how, under 

 these circumstances, the observed deficiency of attraction 

 of the plumb-line towards the mountains would be 

 accounted for. 



.•Mthough this observation upon the plumb-line was not 

 a direct investigation of the force of gravity, it was never- 

 theless conducive to it, for the unexpected abnormality in 

 the horizontal effect of mountain attraction rendered it 

 probable that the same cause, whatever it might be, 

 would produce some corresponding effect upon vertical 

 attraction, i.e. upon gravity. It has been explained how 

 the pendulum is the suitable apparatus for measuring 

 gravity, and accordingly the pendulum w-as called into 

 requisition to make more direct observations. At certain 

 stations of the Indian Survey, of which the height and 

 position had been already determined, the mean number 

 of swings, called the " vibration number," was observ-ed, 

 which were made by the pendulum in twenty-four hours ; 

 and the force of gravity at the different stations \vas thus 

 compared. The local attraction of the elevated mass on 



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