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

Dear G

I began drawing out the machine last night, & I cant help thinking you have got it too big, just think 12″ long inside measure by 7″ in dia^mt.; it will stand 18″ high altogether about. I have also thought of a disadvantage in it; unless the steel rod is in a perfect straight line with the axis of solid cylinder, the wobble of this cylinder will make very great resistance as in my picture. [DIAG HERE] I think the enclosed would be a better plan. It would be very difficult to get a thin steel rod exactly concentric with the brass cylinder this is partly why I think this would be better.

[DIAG HERE]

A is a small ball & socket joint, I have drawn it too big in my picture. The wall is made part of the wheel throgh wh. the stell rod lips. I have made the wheel hanging down so as to get the plane in wh. the strings are passing throgh so that if the two weights wh. turn the wheel are not quite equal there will be nothing wh. will tend to make the cylinder get out of the jar & I think that it would keep upright   It seems to me to return to the old machine that a very small error in the truth of the steel rod & cylinder wd. play old harry with your result; so if possible it would be better to avoid that source of error. Consider the size again with rule in hand & let me hear my orders as soon as possible

Yours | H Darwin