From Horace Darwin to G. H. Darwin 2 April [1875]

Dear George

I have just got your letter, I was talking to Parsons last night abt. the machine, & he suggest what I think is an improved form. I enclose picture. The solid cylinder has a steel rod cast in it to make the pivot, this can easily be done. this rod is turned all one size say $\frac{1}{4}$ in in diamt.

In the boss of the wheel there is a hole through wh. the steel rod is put. The wheel is fixed by means of a set screw A, i.e. a small screw wh. screws into the boss of the wheel and presses against the steel rod; this will hold it very firm. The two bearings in the double brass arm will keep the cylinder steady; and the wheel will take the thrust & prevend the cylinder 2 rising or 1 falling (if you like) by bearing against the double brass arm. Parsons is of opinion that the brass arm had better be supported on both side of the hollow cylinder, I think it would be better. The hollow cylinder might have its bottom cast on; the pointer is fastend as in picture [DIAG HERE] by a set screw. If you think this would do I am game to draw it out properly; it wd. be excellent practice for me. You can find out the resistance of the bottom of the cylinder by fitting it to different depths. or you might replace the cylinder by a disc at end of rod and find its resistance [DIAG HERE]

HD [DIAG HERE]