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

Dear G.

I have just sent off the machine by post; it does not work well because they are fine cells, it was very foolish of us not to think of that before. [DIAG HERE] If O & O′ are the two fixed pts. the only space on wh. the tracer can mark is shaded in my picture, only I have exaggerated it. If you care to work out the best proportion of the cells I will alter those cells; the string stretching is bad to, I think if you are to make it any good you will have to try several sorts of string, such as, fishing tine, thin silk ribbon, thin tape &c. I think you will be doing wrong to read the paper with out having taken the trouble to try it. The needles with one disc on them do very well to hold down the fixed point & the needle with two discs does pretty well for the marker.

The lamps are 2^ft 7ⁱⁿ from end of table measured from inside of the cushion, & 2^ft 8ⁱⁿ high. To make the tracer work you will have to put a piece of paper over one cell & under the other to stop the pins catching   I think a “pitch chain” might do & of course it would not stretch, & pitch chain is a chain like the one in a watch

[DIAG HERE]

wh. perhaps might do but I think it would be too thin. In your letter you do not say whether you were talking bosh or not about inverting force with a P.C & I want to know how you do it if you do. I think your suggestion about the combined wt. & spring very very good only I think a very long piece of elastic would be better.

I dont under stand how you get the inverse square from your P.C.; I send you what I make out of it. Write me a line to say whether you or I am right. The pieces of elastic across the P.C. are no good in the tracing machine, as you want the pieces of elastic to stretch from no length to 6ⁱⁿ.; but it (wh. I ought to have thought of but of course didnt) is easy to keep the string taut by pressing the tracer towards the two fixed fixed points. If you ever get the machine to work well I think I could make it mark in pencil, wh. I think would be nicer, but there would be some little work about it.

H Darwin

[DIAG HERE] I should pass a pin through eye of needle | piece of black lead size of needle | O needle | Rool of paper bound on to needle with string

We have just heard from Lenny you will get in soon.