Laboratory at St. John's College, Oxford. 225 



case projects on each side a horizontal spring, carrying a bit 

 of platinum-foil just under the platinum termination of the 

 screw. These springs are of phosphor-bronze, a material re- 

 commended to me by Dr. Siemens. I dare say other materials 

 would answer; but these have certainly been faultless in my 

 hands ; and, as I have said, the steel was by no means so. 

 The horizontal springs communicate with the wires which are 

 laid to the laboratory. The return-circuit is taken through 

 the gas- and water-pipes. 



This arrangement permits the adjustment for beat to be 

 made with great accuracy. I have watched the dial going 

 with these contacts continually, and never knew them miss. 

 The electric communication, being once laid on from the clock, 

 is of course available for all purposes. 



The arrangement of the electromagnetic dial is simple. Two 

 electromagnets stand opposite each other, and pull alternately 

 at a framework of brass and soft-iron armatures suspended 

 pendulum fashion, within which is a ratchet-wheel of thirty 

 teeth. Two bits of phosphor-bronze spring, attached to the 

 inside of the framework, impel the ratchet-wheel through half 

 a tooth at each second. The spindle of the ratchet-wheel 

 carries a hand which shows seconds on a dial. The same 

 spindle carries an endless screw, which gears in a worm-wheel 

 of sixty teeth. The spindle of this latter wheel carries a hand 

 which shows minutes on another dial. The arrangement, 

 though simple, answers its purpose completely. The only dif- 

 ficulty with it arises in connexion with the use of Leclanche 

 cells : these rapidly lose their strength when set to continuous 

 work of this kind. I have hitherto managed by keeping a 

 number of them and changing those in use from time to time. 

 The whole of this little machine was made in the laboratory. 



The electromagnets were wound, and the wheels and spindles 

 cut, on the lathe. 



Metal-casting Arrangements. — I thought it would be useful 

 to be able to do castings on a small scale; and I employed one 

 of Fletcher's gas-furnaces in the first instance. Brass was 

 given up, on account of the intolerable nuisance caused by the 

 fumes of the burning zinc; but gun-metal worked very well. 

 I procured some moulding-sand, and flasks or boxes of differ- 

 ent forms. I cast a number of gun-metal rings for the spring- 

 chucks that were made for wheels, as also the circles for the 

 small model of the precessional globe. But I found that I 

 should frequently require larger plant ; and considering that 

 I had only the one room, and the mess that is made by the 

 cleaning, wetting, drying, and other treatment of the sand and 

 loam used, 1 thought it best to give up the casting, at all 



