96 REPORT— 1881, 



The box was' placed standing on tbe base of tlie gallows, witb the 

 chimney vertically underneath the round hole in the lintel. The top of 

 the chimney nearly reached the lower face of the lintel, and the iron rod 

 of the pendulum extended a few inches down into ihe chimney. The 

 pendulum wire ran down the middle of the chimney, and the lower half 

 of the pendulum bob was visible through the open face of the iron box. 

 The stone gallows faces towards the S.E., but we placed the box askew 

 on the base, so that its open face was directed towards the S. 



The three legs of the box rested on little metal discs, each with a conical 

 hole in it, and these discs rested on three others of a somewhat larger size. 

 When the box was set ajo^^roximately in position, we could by an arrange- 

 ment of screws cause the smaller discs to slide a fraction of an inch on the 

 larger ones, and thus exactly adjust the position of the box and chimney. 



A small stand, something like a retort stand, about 4 inches high, 

 stood on a leaden base, with a short horizontal arm clamped by a screw on ' 

 to the thin vertical rod. This was the ' fixed ' support for the bifilar 

 suspension of the mirror. The stand was placed to the E. of the pendulum 

 bob, and the horizontal arm reached out until it came very close to the 

 sjDike of the iiendulum. 



The suspension and protection from tarnishing of our mirror gave us 

 much trouble, but it is useless to explain the various earlier methods 

 employed, because we have now overcome these difficulties in a manner to 

 be described later. The two cocoon fibres were fixed at a considerable 

 distance apart on the edge of the mirror, and as they were very short they 

 splayed out at nearly a right angle to one another. By means of this 

 arrangement the free period of oscillation of the mirror was made very 

 short, and we were easily able to separate the long free swing of the pen- 

 dulum from the short oscillations of the mirror. 



The mirror was hung so that the upper ends of the silks stood wiiliin 

 an eighth of an inch of one another, but the tip of the spike stood ^ or -j'|j of 

 an inch higher than the fixed support. The plate-glass front of the box 

 was then fixed on with indiarubber packing. 



It is obvious that a movement of the box pai'allel to the front from E. 

 to W. would bring the two fibres nearer together ; this operation we shall 

 describe as sensitising the instrument. A movement of the box perpen- 

 dicular to the front would cause the mirror to show its face parallel to tlie 

 front of the box ; this operation we shall describe as centralising. As 

 sensitising will generally decentralise, both sets of screws had to be 

 worked alternately. 



The adjusting screws for moving the box did not work very well ; 

 nevertheless, by a little trouble we managed to bring the two silks of the 

 bifilar suspension very close to one another. 



After the instrument had been hung as above described, wo t)-ied a 

 preliminary sensitisation, and found the pendulum to respond to a slight 

 touch on either pier. The spot of light reflected from tlie mirror was very 

 unsteady, but not nearly so much so as in the Glasgow experiment; and 

 we were quite unable to produce any perceptible increase of agitation by 

 stamping or swaying to and fro on the stone floor. This showed that the 

 isolation of the pier was far more sati.sfactory than at Glasgow. 



We then filled the box and pipes with water. We had much trouble 

 with slow leakage of the vessel, but the most serious difficulty arose from 

 the air-bubbles which adhered to the pendulum. By using boiled water 

 we obviated this fairly well, but we concluded that it was a great mistake 



