From Horace Darwin to G. H. Darwin [May 1876 – July 1878]

Dear George,

thanks for your letter about the ram question   i am afraid that it is too complicated ever to be of any use, but i shall have a try to see what can be done with it. i will tell you all about it when i come up for my sunday at cam. this term.

it seems to me that you are not looking at the evaporation question in the right sort of way. it is what function of the readings of the west & dry bulb thermometer & not of the moisture of the air, that frank wants to know that the evaporation from the surface of water is, which seem to me very different questions; what you say about draughts &c will, i think, have the seme effect on the wet & dry bulb as on the surface of water; but ithink that it would be best to find the evaporation from the surface of the wet bulb, in fact i;think thatthis is more what frank wants. this is of course proportional to the quantity of heat that it the wet bulb receives in a unit of time; & this is proportional to the difference of the temperatures, is’nt it; i ca’nt remember, but i have a sort of feeling that the absolute temperature comes in some how.

this comes to newtons law of.cooling i suppose which i never can remember, & i have no books to refer to here will you see if this is correst.

i’m on the mend & soon shall be back to work.

yours, | horace darwin