230 Proceedings of the Royal Society of Victoria. 



that might cause corrosion, oxidation, or usage likely to cause 

 abrasion, has to be most carefully avoided. 



There are two stands : one which is exhibited is the vacuum 

 stand or receiver, in which the pendulums are swung in vacuo or 

 at any atmospheric pressure below the normal ; it is a hollow 

 cone of gun-metal with a wide base, and formed with a flange at 

 the top to receive the dome, which has a similar flange at the 

 bottom. These flanges are ground together, and when greased 

 with tallow form an air-tight joint. The inner part of the flange 

 of the stand carries a strong moveable metal stage, accurately 

 fitted on, which carries the knife-edges, lifting lever, a fixed mirror, 

 and a thermometer. This stage is secured by two strong milled 

 head screws, and can be readily removed for putting in or taking 

 out the pendulums. The requisite attachments for exhausting 

 the chamber, attaching a barometer or manometer, as well as a 

 lever for giving the necessary impulse to the pendulums, are 

 provided ; the latter working through stufling-boxes. The stand 

 in use rests by three studs on a stout tripod with three levelling 

 screws. The receiver and dome with the tripod weigh about 

 fifty-two pounds. " 



The knife-edges, like the agates, are necessarily in two pieces, 

 but they are practically in one by the mode of construction. A 

 block of gun-metal 50mm. wide, 64mm. long, and 10mm. thick, 

 to carry the knife-edges, is strongly screwed on to the platform of 

 the stand, the opposing surfaces being ground together ; the front 

 of the block is 25mm. thick for 10mm. back, and this forms the 

 matrix for the knife-edges ; this front part of the block is 

 divided by a recess to admit the pendulum rod, 20mm. wide, and 

 15mm. front to back. The knife-edges are made of the finest 

 steel "glass hard." The mode of construction was as follows : — 

 The steel prism, from which the knife-edges were eventually 

 formed, was first fitted into a groove on the top of the block which 

 was planed out to the proper form and the prism driven in as a 

 " drift." The prism was then taken out, cut in two and hardened, 

 then the grooves were slightly closed at the upper edges, and 

 the pieces fixed finally in their place. The knife-edges were then 

 ground, sharpened and polished as one piece by means of a special 

 tool. This is a square base of cast-iron in which the gun-metal 

 block carrying the knife-edges can be fixed precisely at right 



