The life of the Hon. Henry Cavendish : including abstracts of his more important scientific papers, and a critical inquiry into the claims of all the alleged discoverers of the composition of water / by George Wilson.

  • George Wilson
Date:
1851
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Licence: Public Domain Mark

Credit: The life of the Hon. Henry Cavendish : including abstracts of his more important scientific papers, and a critical inquiry into the claims of all the alleged discoverers of the composition of water / by George Wilson. Source: Wellcome Collection.

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    MISCELLANEOUS PAPERS. experiments were continued till May, 1842, and the final result was, that the Mean Density of the Earth is 5-6604. It only remains to notice the papers on the Density of the Earth, which have appeared since the publication of Mr. Baily's volume in 1843. In 1847, G. W. Hearn, Esq. read a paper to the Royal Society On the Cause of the Discrepancies observed by Mr. Baily with the Cavendish Apparatus for determining the Mean Density of the Earth. The object of this communication was to draw attention to the probability of the anomalous vibrations of the torsion-rod, being in part occasioned by the magnetic or diamagnetic condition of the masses.* In 1849, Prof, J. D. Forbes re-directed attention to a method of determining the density of the earth suggested by Prof. Robison, by taking advantage of the high tide which rises in the Bay of Fundy, Nova Scotia. The object was to determine the earth's density by the attraction of the tide-wave on a plummet or spirit-level, on the same principle as Maskelyne's experiment on Schiehallien, but with the superior advantages arising from the per- fect homogeneity of the attracting mass, and from the circumstance that all the observations might be made at a single station. The experiment might, in short, appear to unite the advantages both of Maskelyne's and Cavendish's methods of determining the earth's density. Prof. Forbes has made the calculation approximately for an assumed height of the tide-wave. Robison reports the water in Fundy Bay to rise 100 feet at spring tide. Professor Forbes accordingly has calculated the horizontal attraction of a semicylinder of water 100 feet thick, and of about two, four, and eight miles radius upon a point at the extremity of the axis of such a semicylinder. The influence, however, of a tide-wave 100 feet thick, with a radius of 40,000 feet, upon a plumb-line, would produce a deviation of only 0'53 (fifty-three hundredths of a second). Even the greatest of these calculated deviations afi'ords no ground for hoping that the method of Robison could be applied with any success to determine the earth's density.f It is not a little curious that Robison had been anticipated by Caven- dish in suggesting this method of procedure. Among the Cavendish MSS. is a parcel in Cavendish's handwriting, entitled Attraction, and con- taining a variety of packets of papers on various matters connected with the estimation of the earth's density. One of these packets is entitled, Paper given to Maskelyne relating to Attraction and Form of Earth. No. 6. In this paper Cavendish calculates the deviation which the tide in the Bristol Channel would occasion on a plumb-line, on the supposition that the Channel is ten miles broad, and that the tide rises fifty feet; and comes to the conclusion that the mass of water would make the plumb- line deviate If seconds, if the mean density of the earth is the same as that of the surface. (P. 9.) It further appears that Boscovich has pre- ceded Cavendish; for the latter, in another part of the paper, says, Since I saw you [Maskelyne], I have looked again into Boscovich's book (' De Littoraria Expeditione, &c.'), and find that he supposes the arm of the sea to be 100 miles broad, in which case he says the plumb-line will deviate 2 38'. (P. 10.) From these passages it will be seen that Boscovich, Cavendish, and Maskelyne were aware of the method gene- rally believed to have been suggested by Robison. I do not know the exact date of the communication of Cavendish to Maskelyne, but a letter * See Report in Athenmum, March 27th, 1847. t The extracts in tlie text are taken from Prof. Forbes's interesting Note rer/ardinn an exjienmenl suggested by Professor Robison. (Proceed, of R.S.E., vol. ii. (1849) p. 244.)
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