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Darwin Correspondence Project

From G. H. Darwin   16 October 1873

Trinity College | Cambridge

Wedn. Oct. 16. 73

My dear Father,

Your problem I understand to be this1


Drop a marble down AB on to the inclined plane BC,—find the inclination of BC to the horizon so that the range BC may be a maximum. If the marble is perfectly elastic the inclin. is 30o to horizon or 60o to the vertical; if of glass (on to a glass plane) it is 29o; if of tight packed wool or of iron it is 21o. And the less elastic the ball & plane are the less must be the inclin. to the horizon2

I have no conception what the elasticity of water on a leaf would be; I fancy it is rather the upper part of the drop wh. splashes off the lower part & then what we require is the elasticity of water on water. From the height wh. water splashes I shd. think the elasticity must be pretty great. I am thinking however of trying an experiment wh. may give a clue—by squirting ink out of a very fine syringe onto inclined sheets of paper & observing the pattern. I have got a syringe but it is too coarse.

Judging from the marble I shd. guess the leaf shd. be3

CD annotations

Top of letter: ‘Draw Leaf | Get Leonard4 to measure angles—’ pencil
End of letter: ‘inclined a little less than 20o degrees to the Horizon.’ pencil


In the diagram, the dotted lines (solid in the original), ‘30’, and ‘60o’ are CD’s pencil annotations.
CD had asked George what angle a waxy leaf ought to hold to the horizon in order that raindrops should bounce off as completely as possible (letter to G. H. Darwin, 12 October [1873]).
CD completed the sentence in his annotation at the end of the letter; he evidently discarded the remaining page or pages of the letter.
Leonard Darwin.


On bodies of varying elasticity bouncing off inclined planes [see 9096].

Letter details

Letter no.
George Howard Darwin
Charles Robert Darwin
Sent from
Trinity College, Cambridge
Source of text
DAR 162: 65
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 9097,” accessed on 20 April 2018,

Also published in The Correspondence of Charles Darwin, vol. 21