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Darwin Correspondence Project

To Leonard Horner   [January 1847]1

Down. Farnborough. Kent.


My dear Mr Horner.

Your most agreeable praise of my Book2 is enough to turn my head; I am really surprised at it, but shall swallow it with very much gusto.

With respect to the inclination of the lava-streams, it would indeed as you say be utterly impossible to measure them with any clinometer. The river was carefully mapped, so that the distance between any two points was known: then I measured at (A) the height of lava-stream above the river & the fall of the river having been ascertained the height above the sea could be approximately known.

Then the same operation being done at B, the difference in height made a triangle & the angle is most easily calculated by trigonometry. [DIAGRAM HERE] angle to be ascertained B A slope of river sea-level Being out of practise my Brother calculated them.3 As I say, these measurements though not absolutely correct, from the data not being so, cannot be far wrong.

E. de Beaumont measured the inclination with a sextant & artificial horizon, just as you take the height of the sun for latitude.—4

With respect to my Journal,5 I think the sketches in the second edition are pretty accurate; but in the first they are not so, for I foolishly trusted to my memory, & was much annoyed to find how hasty & inaccurate many of my remarks were, when I went over my huge pile of descriptions of each locality.—

If you ever meet anyone circumstanced as I was advise them not on any account to give any sketches until his materials are fully worked out.

With many thanks for the pleasure your commendations have given me, believe me | Dear Mr Horner | Yours very truly | C. Darwin.

What labour you must be undergoing now; I have wondered at your patience in having written to me two such long notes. How glad Mrs. Horner will be when your address is completed.6

I must say that I am much pleased that you will notice my volume in your address, for former presidents took no notice of my two former volumes.7

I am exceedingly glad that Bunbury is going on well.—8


The copyist recorded that the original letter was marked ‘C. Darwin Jan 1847’.
South America, published in October 1846 (see Correspondence vol. 3, letter to Leonard Horner, [23 December 1846 – January 1847]).
Erasmus Alvey Darwin. See Correspondence vol. 3, letter from E. A. Darwin, [May 1844 – 1 October 1846].
The reference is to CD’s discussion of lava streams in the Santa Cruz river valley, Patagonia, in South America, pp. 115–17, where he refers to Jean Baptiste Armand Louis Léonce Élie de Beaumont’s measurements of the least inclination of the upper surface of a lava stream.
Journal of researches 2d ed.
As president of the Geological Society, Horner was preparing his anniversary address (Horner 1847).
The retiring presidents of the Geological Society in 1843 and 1845 were Roderick Impey Murchison and Henry Warburton respectively.
Charles James Fox Bunbury, Horner’s son-in-law, was recovering from measles (see Correspondence vol. 3, letter to Leonard Horner, [23 December 1846 – January 1847], n. 6).


Responds to LH’s comments on South America.

Discusses inclination of lava stream.

Sketches in second edition of Journal of researches more accurate than in first.

Letter details

Letter no.
Charles Robert Darwin
Leonard Horner
Sent from
Source of text
DAR 145: 139
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 1048,” accessed on 23 March 2019,

Also published in The Correspondence of Charles Darwin, vol. 4