From Horace Darwin to G. H. Darwin [19 April 1879]

Dear George,

Sir W.T. asked me to ask you to remember him to Mr McLennan when you wrote to him, & Lady T asked me to tell you that they will be at Glasgow till May 20^th., but go to Edinburgh for one day viz. May 13^th. & that they will be very glad to see you any time. Now I will give you an account of my doings. I found everything going on very well at Whittinghame, so I only stayed there Tuesday & Wednesday night. On Wed morning I heard from Sir W. T. saying he should be very glad to see me any day this week & asking me to sleep; so I telegraphed back to say that I would come that night. I found he lived in the College in a very nice house; he received me very cordially, & so did Lady T. There was a party of students that night so I did not hear much about the moon; but next morning after his lecture I went over to the laboratory, & he telephoned to White—the instrument maker—to make a pendulum at once. It was to consist of a fine brass wire, to the bottom of wh. a lead wright 3″ long by 1″ diameter was soldered; this was the bob, and it was turned true & the wire fixed centrally. The top end of the wire—wh. was 1 metre long—was soldered into a brass plate, hence the pendulum could only move by bending the fine brass wire. This came up at about 2 o’clock & was fixed to a stone pier in the laboratory; this pier was built on separate foundations, but as we found out afterwards was not quite unconnected with the floor; I think the boards touched it. From the underside of the lead bob a pin projected about $\frac{1}{4}$″ to which one of the silk fibres of the bifilar suspension was fixed. This was fixed up by 7 oclock & the lamp & scale fixed, the

[DIAG HERE] [diagram of a machine, annotated with "bob of pendulum", "sheet copper held down on board by a weight", "silk", "mirror", and "board fixed"]

[DIAG HERE] [diagram of a machine annotated with "brass plate to which pendulum wire is soldered", "stone pier", "bob of pendulum", and "mirror"]

whole thing enclosed in an air tight case with a glass window and lense so that it worked well, but the spot of light jumped about like anything. Then we went in to dinner, there was a dinner party, James Thomson, Mr Jeb, & lot more. At about 12 we went back & still found the spot of light moving, though not nearly so much, it kept on the scale now. It some times got pretty quiet, but never quite still, never still enough to take a reading. Sir W.T. think it was the earth shaking, though sometimes moving about the room certainly made is move, & treading on the board next it, which I think touched it, moved it very much.

I tried pushing the stone pier, quite gently with one hand, this made it swing so much that it did not recover it for $\frac{1}{4}$ hour. It was very hard to believe that my push moved that large block of stone.

Sir W. T thinks that we can get lunar gravity unless we get something biger and as important. He proposes hanging it in liquid to damp it. He is going to observe this pendulum a bit more & write to you. You ought to go down there, as I suppose you will; if you do go I must see you first. I think I shall come down to Cam pretty soon & tell you the rest. If we had the pendulum 250′ down the well & got the same movement the spot of light would move some miles I should think. Sir W. T. says that the whole pendulum must be in the liquid, so as not to have the surface cut by the pendulum rod, this of course wd. be impossible with 250′ pend^lm., & would make the adjustment rather awkward in any case. I told him of my plan of measuring the value of the movement of the spot of light; it is this. AB main pendulum CD very light pendulum, bob weighing 1 grain, supported by silk fibre; the little bob C is connected [DIAG HERE] to big bob B by silk fibre

If C is moved horizontally, this will put a tension on silk fibre which connects B & D.

suppose B= 10000 grs = 10⁴

D= 1 gr.

C move horizontally $\frac{1}{\mathrm{10\; 2}}$ millimetre one can read to this with micrometer.

T= tension of fibre BD

T= $\frac{1}{\mathrm{10\; 2}}$ $\frac{1}{\mathrm{10\; 3}}$ = $\frac{1}{\mathrm{10\; 5}}$

^T/wrobQQQQ B^ = $\frac{1}{\mathrm{10\; 5}}$ $\frac{1}{\mathrm{10\; 4}}$ = $\frac{1}{\mathrm{10\; 9}}$

which is small enough. Sr. W.T. told me that he had a piece of apparatus like this for measuring small horizontal forces, & he thought it wd. be the best way.

Yours ever | H Darwin