Record

HSF - Correspondence of Sir John Frederick William Herschel, and the Herschel family
- 2 - Correspondence of Colonel John Herschel
  - 1 - Letters of John Herschel to Margaret Brodie Herschel and John Frederick William Herschel

RefNo	HSF/2/1/111
Previous numbers	32.8.97 and 32.8.97A-C
Level	Item
Title	Letter from J [John] Herschel, Camp, to 'Dear Pater' [Sir John Frederick William Herschel]
Creator	Herschel; John (1837-1921); British army officer and astronomer
Date	24 December 1870
Description	His father's letter of 17 November finds him working from the plan discussed. They are in accord apart from on the scale of distribution of local stations. It is impossible to deal with the Earth as a whole, John thinks. Even if European triangulations could be united, those of other continents are separate and two-thirds of the surface is niot susceptible to taking any part. He discusses the shape of the Earth as a spheroid, or ellipsoid; it is not clear from the partial data whether the latter conclusion is real and he explains the lack of clarity in his mind. They must start with some manageable regular figure, however. They start on the problem of local attraction with the hypothesis of an ellipsoid with know elements and he cites [Alexander Ross] Clarke's approximations for the comparison of geodetic and astronomical amplitudes. He discusses problems of figure and local attraction, and the difference between the physical form of the surface and the imaginary one which is the direction of the plumb line at any point. The real question, he thinks, is what are the irregularities of the attraction-surface? The asnwer can only be gained by numerous measurements of the deviation distributed according to practical experience, John believes, and he lays out a methodology for doing this. He thinks what is needed is a systematic investigation of the effcets of local attrcation within a limited area of ground free from 'superincumbent masses', and he discusses this idea. Colonel [James Thomas] Walker will not leave England without the two transit instruments in Strange's hands, intended for this work. He notes the system for placing latitude stations at every degree alond adjacent meridians, due to Walker's respect for Captain Clarke, who wrote about it long ago, but who was misapprehended. John discusses this, concluding that he needs to know how likley it is that that the work he is doing will lead to a result. If the pendulums go home without playing their part, a great opportunity would be lost. He has been told that local attraction is of small interest, to which he replies that it is a source of error of unknown magnitude; ignorance of which he thinks may have led to an enormous waste of energy. John describes his current work on the group system, and offers a justification for his transit scheme, thinking that his plan is not quite understood. He explains this in more detail. He does not have the Royal Society [membership] list with him in camp but there are few names he could mention. Mr [George Biddell] Airy's signature would be useful, but he does not know how to ask for it. Mr [John Edward] Gray is a friend of Mary's [Mary Cornwallis Herschel] and she thinks he would be pleased to be asked [to sign the Fellowship candidature certificate].
Extent	16p.
Format	Manuscript
PhysicalDescription	Ink on paper
AccessStatus	Open

Fellows associated with this archive

Code	PersonName	Dates
NA7955	Herschel; John (1837 - 1921)	1837 - 1921

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