ON THE ELASTICITY OF -WIRES. 



61 



Fig. 6. 



exhausted glass tube (to prevent oxidation of the gold contacts) and when 

 required for use is placed in the clock case just below the iron ' bob ' of 

 the pendulum. 



As the pendulum swings the magnet answers 

 to its motion and draws the forked lever into 

 contact with either of the screws s s' which are 

 tipped with gold. The buckling of the glass 

 fibre tends to make the contact very cei'tain 

 and avoids any danger of recoil, while there 

 being no oxygen left in the tube there can of 

 course be no oxidation of the contacts. 



It is supposed that the clock has a mercurial 

 pendulum with cast-iron cistern, as most pendu- 

 lums are now made. 



The above arrangement is not theoretically 

 perfect, for there must of course be some slight 

 reaction from the magnet to the pendulum ; but 

 as the pendulum weighs, or should weigh, about 

 forty pounds and the magnet about ten grains, 

 the reaction must be very slight, and even this 

 would be of no consequence provided the mag- 

 netisation of the needle remained constant. 



The convenience of the arraugement, and 

 the ease with which it can be applied without 

 interfering or tampering with the clock, com- 

 mends it for practical work. 



The only practical fault I see in Mr. Gill's 

 arrangement for driving the pendulum, is the 

 extremely small ' travel ' which the impulse lever 

 has in each impulse. This will necessitate very perfect 'banking ' arrange- 

 ments, for a very small difference in this travel will make a large difference 

 in the impulse on the pendulum, and the perfection of the arrangement 

 depends on the impulse being a constant. It appears to me that it would 

 be desirable to make the impulse-arm very light, but longer in its travel, 

 and acting perhaps farther down on the pendulum rod. 



These are the only points that occur to me. 



Faithfully yours, 



Ho WARD GeUBB. 



Dublin : August 23, 1880. 



'rrrrrrt^ ' 



'mrmnrv 



Report of the Committee, consisting of Professor Sir William 

 Thomson, Professor Tait, Dr. C. W. Siemens, Mr. F. J. Bram- 

 WELL, and Mr. J. T. Bottomley (Secretary), for commencing 

 Secular Experivients on the Elasticity of Wires. 



The Committee have but little to add to their reports of the last years. 

 The arrangements in the tower of Glasgow University may now be 

 regarded as complete, so far as concerns the wires already suspended 

 there for experiment. At the last meeting of the Association it was 

 reported that pairs of wires of gold, platinum, and palladium had been. 



