HOGG. — FRICTION AND FORCE DUE TO TRANSPIRATION. 125 



the centre about 0.075 cm. in diameter, through which passes the 

 closely fitting supporting wire, B, Figure 2. The latter is provided 

 with a hard-soldered shoulder, against which the plate is clamped 

 by means of the nut below. Platinum foil washers are placed between 

 the glass and the nut. On the supporting wire at C are fixed three 

 feet, long enough so that, if the swinging plate were lowered, they 

 would reach the upper surface of the upper plate just before the 

 suspended plate could touch the lower plate. This is a measure of 

 safety, as will be seen when the process of making and suspending 

 is described. At the proper height for the window, D, is fastened a 

 mirror of platinized glass. It is supported on the wire by thin platinum 

 foil, which is fused to the wire, and then folded around the edges of 

 the mirror. At the end of the wire is a clamp, consisting of two flat 

 pieces of platinum 1.25 mm. thick, 3 mm. wide, and 6 mm. long. One 

 is hard-soldered to the wire so that one surface is in the axis of the 

 wire. The other is fastened upon this by tw^o screws. The fibre is 

 fastened at the top in the similar clamp, F, at the end of the wire, I. 

 The nut, H, into which the wire, I, is screwed, carries a cross-bar, G, 

 the ends of which are engaged by the platinum wire hooks, E and E, 

 shown detached at E' and E'. There is also a check-nut, K, on the 

 wire, I, above H, which can be screwed down upon H to hold the clamp 

 and cross-bar in any desired relative position. The upper end of the 

 wire, I, screws into a nut of the same material as itself on the soft 

 iron armature, J, which is supported by the swivel-head, K. The 

 supporting cross-bar is itself fastened to the two vertical threaded 

 pieces passing through the supports, L L, and furnished with nuts for 

 support and adjustment. 



As the telescope and scale were to be used in observing, and as the 

 whole apparatus was to be heated, a plane parallel window, D, which 

 would bear a temperature of 300° C, must be secured. It was decided 

 to close in the end of a glass tube, and grind the inside and outside 

 flat and parallel to each other, and then seal the short horizontal tube 

 at the proper place to the long vertical one. By using a large lump of 

 optical glass, and using the oxygen flame, it was found possible to close 

 the end with glass of fairly uniform density. In ordinary glass tubes, 

 the streaks in the glass cause difticulty, and besides, almost invariably 

 a small sort of pit is formed at the centre in the process of sealing. 

 At this point, the density of the glass is different from that of the 

 rounded part, thus giving an irregular lens effect. The method of 

 grinding is to use a ring tool with which to make a groove around the 

 inside just as large as the size of the tube will allow. The groove 

 allows clearance for the emery when the process of grinding flat is 



