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Darwin Correspondence Project

From G. H. Darwin   23 October 1877

Trin Coll. Camb

Oct 23. 77

My dear Father,

I have been to see Maxwell today & got his answer to yr. question, which agrees with what I conjectured.1

A thin film of air entangled in a rough surface which water does not wet, will prevent the body under it from drying so rapidly as it wd. otherwise do. Through that film the vapour can only pass by diffusion, whereas if the moist surface were bare, convection would come into play; more or less fresh dry air would be brought near the surface & into this the vapour wd. pass much more rapidly. He illustrated it thus   A field covered with dry stalks of corn will keep the ground moist much longer than without the stalks because the air entangled in the stalks gets moister than the rest of the air & will keep the dryer air from access to the moist surface of the ground.

If however you were to replace the stalks by lamp wicks which suck up the water by capillary attraction then there is a continual subtraction of water from the ground which wd. evaporate easily from the tops.

There is not however more evaporation from a corrugated moist surface than from a flat equally moist one.

Thus a hygroscopic bloom on fruit would not keep the fruit dry, nor yet would it evaporate more rapidly than a smooth surface. I think this meets your views exactly.

My own work goes on excellently & I think I shall make a really good hit with it; I have got to do some fearfully hard work yet & I can hardly think of anything else.2

The proposal for yr. LLD comes on at the next meeting of the Council of the Senate & I believe will be voted on on Thursday, but I don’t suppose there will be any opposition.3 I shd. think they wd. propose some day towards the end of Nov & it will be a Thursday anyhow.

When does Jim come—4 Tell him Dew has got a telephone.5

Your affectionate son | G H Darwin

CD annotations

Top of letter: ‘Evaporationred crayon


CD had sent a query to James Clerk Maxwell via George Darwin about the effect of bloom on the speed with which leaves dried (see letter to G. H. Darwin, 18 [October 1877] and enclosure).
George was calculating the effect of tides in the body of the earth in order to challenge estimates of the age of the earth based on the supposed retardation of the earth’s rotation due to tidal friction; his work was published by the Royal Society of London the following year (G. H. Darwin 1878).
The University of Cambridge intended to confer an honorary degree of doctor of laws (LLD) on CD (see letter to Edward Atkinson, 9 June 1877). The Senate is the governing body of the university.
‘Jim’ was Horace Darwin’s nickname. Horace wrote to George in a letter dated 24 and 25 October 1877, saying that he intended to visit Cambridge in a couple of weeks’ time (DAR 258: 864).
Albert George Dew-Smith was an instrument maker who went into partnership with Horace Darwin in 1878 to form the Cambridge Scientific Instrument Company. Much interest in the telephone was generated by its display at the Plymouth meeting of the British Association for the Advancement of Science in August 1877; it had been first (unsuccessfully) demonstrated in Britain at the 1876 Glasgow meeting of the British Association (Arapostathis and Gooday 2013, p. 99).


Loss of water from leaf surfaces; action of a still air layer.

Proposal for CD’s LL.D.

Letter details

Letter no.
George Howard Darwin
Charles Robert Darwin
Sent from
Trinity College, Cambridge
Source of text
DAR 162: 66
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 11200,” accessed on 27 October 2021,