The Kew vacuum apparatus is built up of successive cylinders of glass, the tops and bottoms of which are ground plane. They are simply put together with hog’s lard. It was said not to be very tight. I once considered making my apparatus in this way.

The Vacuum receiver of Captain Basevi is extremely tight, much better than mine in that respect. My leakage is almost wholly at the windows and his windows are constructed almost precisely like mine. The only difference is that in his  there is a frame of four separate pieces of metal over the glass. It may be that these are screwed down so as to press the panes on with considerable force. I must have such plates fitted to my windows.

Basevi’s receiver is made of copper instead of iron as mine is. Apparently some magnetical effect was feared but of what kind I did not learn. I heard from the Repsolds that Sawitsch who at one time contemplated a vacuum chamber for the reversible pendulum feared that if made of iron there might be a magnetical effect on the knife-edges. This suggests as an easy precaution always setting up pendulums facing north and south. I think, however, the fear of using iron was rather a vague one. Perhaps it is wise however.

The top of Basevi’s receiver instead of being a metallic plate fastened on by bolts is simply a glass

Basevi’s starting lever and also the one which raises the pendulum off the agates (which I do not have at all) are inserted in the side of the receiver so that they are not disturbed by taking off and putting on the cover.

Basevi has no frame which contains the whole apparatus within the receiver and which can be all taken out at once. In this respect I certainly have a better arrangement.

Basevi’s receiver instead of resting simply on feet is attached by a massive iron collar (he does not seem to have feared iron here) about the top to an immense wooden framework. I really can see no advantage in this. What seems to be feared is a flexure of the apparatus as the pendulum swings back and forth. If a great metallic cylinder –the receiver- is going to do this I believe that any wooden frame would do so. Professor Stokes said he had the impression it was originally intended to have two brick piers built at every station where the pendulum was swung.

All the pendulums at Kew except Captain Kater’s reversible pendulum were of one construction. There was a heavy bar of brass with a disk-shaped bob and a very long and large pointer. The knife-edges were however in several cases in quite a shocking condition.

Foster’s agates were not plane but were slightly cylindrical, the axes of the cylinders being perpendicu-

The English pendulums all had two separate agates, ground into one plane (when they were plane). Becker’s balances also have two but the best balances in Germany I understand have only one.

Decidedly, I must invent some way of making experiments on the friction of knife-edges, as the subject is very obscure.

I had several important conferences with distinguished mathematicians and physicists in England in regard to the subjects of my work especially with Maxwell and Stokes.

I called upon the astronomer royal whom I wished to see not only on account of my having made pendulum experiments to determine the density of the earth but also because he had been rather polite to me when I was first in England. But this time he absolutely refused to see me without any explanation, although I wrote on my card who I was. I mentioned the circumstance several times in company in England and the only explanation offered was that his manners were known to be strange. I always added that it was possible he felt bitterly having been forced to go in to the observation of the ’70 eclipse by public opinion after my father’s action, and

The computations were resumed as soon as Mr. Farquhar recovered from his attack, although I have not been able to do any actual numerical work and do not expect to do so. Fair progress was made for about a fortnight.

On the 27th I arrived in Hamburg and went at once to Repsold’s where he had my instrument all set up ready for my inspection. I occupied three days in the examination of all its parts and today I went and accepted it and paid for it. I enclose a photograph of it. There are a number of features of it, I confess, which my judgment cannot altogether approve. Swung in air, as it is, its proportions are not the best possible for eliminating the effect of resistance. But then I cannot admit that it will do to swing in air. Yet here in Germany & Switzerland they do not even read the barometer during the experiments.

the top of which will be closed not by an ordinary piece of plate glass but by a piece of planed glass. The circle at the top to have a diameter of only 2 ½ inches. I propose to set up the apparatus by digging a hole in the ground & setting in it a tight barrel; then the receiver will go into that and will rest by the wings of its collar directly on blocks on a level with the ground. In the side of the receiver will be a lever which shall let the pendulum down onto its agates and also set it swinging. The lower part of the pendulum is to be illuminated either by a Geissler’s tube inside

I will conclude this report by mentioning some matters of general scientific interest. It really seems as if Mr. Crookes had discovered a new force. You have doubtless heard of his experiments according to which radiant light falling on an absorbing surface produces pressure. Mr. Tyndall is warmly of the opinion that this is due to a longitudinal disturbance of the ether. He Tyndall says Stokes also thinks so.

Mr. Lockyer at the mint showed me some new spectroscopic experiments by which he seems to have made out that each metal has four successive espectra according to the intensity of the heat. He and Mr. Clifford think that on this basis they are going to distinguish between four different kinds of heat, produced by four different kinds of motion.

I also saw at the mint a spectroscopic method of quantitative assays which seemed very accurate.

The arctic expedition is to take a number of Wheatstone’s polariscopic clocks. He showed them to me and it was surprizing to see that they would give the time correct to two minutes. M. Cornu was in London while I was there shewing his apparatus for measuring the velocity of light by Fiseau’s method. I was glad to make his acquaintance as it may be useful to me in swinging in Paris. The new tough glass is wonderful. It is in a great state of strain which makes it optically useless and I fancy it cannot be ground without crumbling. Mr. Sylvester is developing a new and important branch of mathematics which has sprung from Peaucellier’s discovery of Parallel motion link-work.

Yours very respectfully

C. S. Peirce

Asst