From Horace Darwin to G. H. Darwin 26 February 1875

Dear G.

I have just got your letter forwarded on here. If you dont want these P.Cs. I should like to keep them, as the seem to work well. & were a great deal of trouble to make. As you still do’nt answer my question about the original thing about inverting force with a P.C. (i.e. if you made a spring with force & as sq. of distance to get from the spring & P.C. a force varing inversely as square of distance), I suppose you are wrong; but I cannot think why you should mind saying so.

[DIAG HERE]

It still seems to me that you are wrong if I understand you construction right, viz. a knitting needle going through two rings at A. & B.. & string going round it & fastened to R & to other end a weight

Now the calculation you send me is what I have done & get the same result. when there is simply a force acting at P. & sent it you.

But it seems to me that you have left out the reaction of the knitting kneedle at A & B. Is this so or is it not. Please answer me this.

H Darwin