# 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

## Footnotes

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.

## Summary

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

## Letter details

Letter no.
DCP-LETT-9097
From
George Howard Darwin
To
Charles Robert Darwin
Sent from
Trinity College, Cambridge
Source of text
DAR 162: 65
Physical description