ON THE MEASUREMENT OF THE LUNAR DISTURBANCE OF GRAVITY. 105 



legs attached is set down so that the inverted V grooves in the legs rest 

 on the rounded screw-head at the tops of the three feet, and each of the 

 feet rests on its three knobs on the stone. The bottom of the copper tube 

 is thus raised 5^ inches above the stone. By this arrangement the copper 

 tube is retained in position with reference to the stone, and it will be 

 observed that no part of the apparatus is under any constraint except 

 such as is just necessary to geometrically determine its position. 



The screws with rounded heads which form the three feet are 

 susceptible of small adjustments in height, and one of the three heads is 

 capable of more delicate adjustment, for it is actuated by a fine screw, 

 which is driven by a toothed wheel and pinion. The pinion is turned by 

 a wooden rod, made flexible by the insertion of a Hook's joint, and the 

 wooden rod reaches to the top of the tub, when it is mounted surrounding 

 the instrument. 



The adjustable leg is to the N. of the instrument, and as the 

 mirror faces S. we call it the 'back-leg.' When the copper support 

 is mounted on its three legs, a rough adjustment for the verticality of 

 the tube is made with two of the legs, and final adjustment is made by 

 the back-leg. 



It is obvious that if the back-leg be raised or depressed the point of 

 the pendulum is carried southwards or northwards, and the mirror turns 

 accordingly. Thus the back-leg with its screw and rod affords the 

 means of centralising the mirror. The arrangements for suspending the 

 mirror must now be described. 



The lower plug in the pendulum-bob is rounded and has a small 

 horizontal hole through it. When the pendulum is hung this rounded 

 plug just appears through the hole in the diaphragm at the bottom of 

 the copper tube. 



A small brass box, shaped like a disk, can be screwed on to the bottom 

 of the copper tube, in such a way that a diameter of the box forms a 

 straight line with the axis of the copper tube. One side of the box is of 

 plate glass, and when it is fastened in position the plate glass faces to 

 the S. This is the mirror-box; it is of such a size as to permit the 

 mirror to swing about 15° in either direction from parallelism with the 

 plate- glass front. 



The fixed support for the second fibre for the bifilar suspension of the 

 mirror may be described as a very small inverted retort-stand. The 

 vertical rod projects downwards from the underside of the diaphragm, 

 a little to the E. of the hole in the diaphragm ; and a small horizontal 

 arm projects from this rod, and is of such a length that its extremity 

 reaches to near the centre of the hole. This arm has a small eyelet-hole 

 pierced through a projection at its extremity. 



The mirror itself is a little larger than a shilling and is of thin plate 

 glass ; it has two holes drilled through the edge at about 60° from one 

 another. The mirror was silvered on both sides, and then dipped into 

 melted paraffin ; the paraffin and silver were then cleaned off one side. 

 The paraffin protects the silver from tarnishing, and the silver film seen 

 through the glass has been found to remain perfectly bright for months, 

 after having been immersed in fluid during that time. A piece of platinum 

 wire about xoVw^t of an inch in diameter is threaded twice through each 

 hole in opposite directions, in such a manner that with a continuous piece 

 of wire (formed by tying the two ends together) a pair of short loops are 

 formed at the edge of the mirror, over. each of the two holes. When the 



