104 KEPOBX — 1881. 



whilst the central part was left cylindrical. The upper plug with its 

 pulley is replaced by another pliig bearing a short round horizontal rod, 

 with a rounded groove cut in it. The groove stands vertically over the 

 centre of the weight, and is designed for taking the wire of the bifilar 

 suspension of the pendulum ; when riding on the wire the pendulum-bob 

 hangs vertically. 



Part of this upper plug consists of a short thin horizontal arm about 

 an inch long. This arm is perpendicular to the plane of the groove, and 

 when the pendulum is in position, projects northwards. Through the 

 end of the arm is bored a fine vertical hole. This part of the apparatus 

 is for the modified form of disturber, which we are now using. 



The support for the pendulum consists of a stout copper tube 2^ 

 inches in diameter inside measure, and it just admits the pendulum-bolj 

 with l^th inch play all round. The tube is 3 feet G inches in height, and 

 is closed at the lower end by a diaphragm, pierced in the centre by a 

 round hole, about ^ inch in diameter. The upper end has a ring of ,brass 

 soldered on to it, and this ring has a flange to it. The upper part of the 

 brass ring forms a short continuation 4' of an inch in length of the 

 copper tube. The ring is only inti'oduced as a means of fastening the 

 flange to the copper tube. 



The upper edge of the brass continuation has three V notches in it at 

 120° apart on the circumference of the ring. A brass cap like the lid of 

 a pill-box has an inside measure ^ inch greater than the outside measure 

 of the brass ring. The brass cap has three rods which project inwards 

 from its circumference, and which are placed at 120° apart thereon. 

 When the cap is placed on the brass continuation of the upper tube, the 

 three rods rest in the three V notches, and the cap is geometrically fixed 

 with respect to the tube. A fine screw works through the centre of the 

 cap, and actuates an apparatus, not easy to explain without drawings, by 

 which the cap can be slightly tilted in one azimuth. The object of 

 tilting the cap is to enable us to sensitise the instrument by bringing the 

 silk fibres attached to the mirror into close proximity. 



Into the cap are soldered the two ends of a fine brass wire ; the 

 junctures are equidistant from the centre of the cap and on opposite sides 

 of it ; they lie on that diameter of the cap which is perpendicular to the 

 axis about which the tilting can be produced. 



"When the pendulum is hung on the brass wire loop by the groove in 

 the upper plug, the wires just clear the sides of the copper tube. 



It is clear that the tilting of the cap is mechanically equivalent to a 

 shortening of one side of the wire loop and the lengthening of the other. 

 Hence the pendulum is susceptible of a small lateral adjustment by 

 means of the screw in the cap. 



To the bottom of the tube is soldered a second stout brass ring ; this 

 ring bears on it three stout brass legs inclined at 120° to one another, 

 all lying in a plane perpendicular to the copper tube. From the extre- 

 mity of each leg to the centre of the tube is 8^ inches. The last inch of 

 each leg is hollowed out on its under surface into the form of a radial V 

 groove. 



There ai-e three detached short pieces of brass tube, each ending below 

 in a flange with three knobs on it, and at the upper end in a screw with a 

 rounded head. These three serve as feet for the instrument. These 

 three feet are placed on the upper surface of our basement stone at 120° 

 apart, estimated from the centre of the stone. The copper tube with its 



