From Horace Darwin to G. H. Darwin [April 1875]

Dear G.

I think the little cylinder had better be all made of steel; (in one piece of) (& then it must be true) the foreman says that [DIAG HERE] if so it will turn up truer & cost very little more. I’m getting on well with the picture & shall have finished it early next week; would you like to see it when done. I can easily send it by post. I have got the spindle 3/8ⁱⁿ in diameter. In your designe you [DIAG HERE] have got the distance (a b in last picture) of small cylinder from bottom of vessel 3/4ⁱⁿ, I think that is rather near, but you know best; the reason I think it near is only that it will increase the error from the shearing of the bottom of small cylinder. What do you say to having the dia^mt. of small cylinder 3/4ⁱⁿ instead of 1ⁱⁿ? this will also reduce the error of bottom surface. They will be able to turn it with great acuracy, to at least with-in the $\frac{1}{100}$ of an inch, & much near than that I think. They will turn it so that a guage in the form of a ring will just slip over it. I’m sorry to trouble you so often, but I’m inclined to think that 3/4″ wd. be better than 1″ for diamt of cylinder & 1ⁱⁿ better than 3/4 for distance   The more I think of my last night scheme the more I think it will act.

H D.