106 Prof. Bichard Threlfall on the 



all temperatures up to 98° C, and at pressures ranging from 

 76 to one or two centimetres of mercury. 



A cube of brass, measuring about four inches on the side, 

 was bored through the centre of one face to a depth of about 

 three inches, the diameter of the bore being about one inch. 

 This block served as a support to the vibration-tube, which was 

 jacketed and projected into the hole in the block. The tube 

 was provided with a flange so that it could be connected air- 

 tight to the block. A hole about one inch in diameter, and 

 whose axis was horizontal when the vibration-tube was vertical, 

 was cut through the block and opened into the bore. This 

 hole was glazed with carefully selected glass and allowed the 

 motion of the vibrator to be observed. In order to get the 

 window air-tight, the necessity for considerable pains was 

 anticipated and experienced. Rubber washers, coated with 

 white lead, and compressed against the glass by a good and 

 powerful screw, enabled me finally to dispose of the difficulty. 

 Additional light was let into the cavity in the block by means 

 of a solid glass rod running horizontally into the block at an 

 angle of about 40° to the axis of the window-hole. This bar 

 was secured by screw-packing in the usual manner. Every- 

 thing was straight and all the parts true, so that the vibration- 

 tube could be made truly vertical by setting the brass block 

 horizontal. This was done in the usual way. The vibration- 

 tube was about 24 inches long, and was furnished with 

 a small side tube so as to allow of its being put in connexion 

 at will with a Bunsen pump, a Sprengel pump, and an air- 

 drying apparatus. The jacket was furnished in a similar 

 manner with an inlet and outlet for steam or other vapour, and 

 was wrapped in cotton-wool and paper to a thickness of about 

 half an inch all round. The ordinary brass-rod suspender 

 was passed through a rubber bung, fitting the top of the 

 vibration-tube, and surrounded by a movable circular wooden 

 trough which could be filled with mercury, and thus form a 

 mercury seal for the bung. In spite of all the trouble taken, 

 the apparatus was never absolutely air-tight, the high tem- 

 perature at which it was used preventing me from reaping 

 the advantage of covering doubtful places with marine glue. 

 The vibrators used were (1) : — A pure silver anchor-ring sup- 

 ported by a disk of aluminium foil, through the centre of which 

 passed a short bit of aluminium wire to which the thread was 

 attached. (2) A cylindrical vibrator of brass, vibration being 

 about the axis of the cylinder. (3) A similar but larger 

 vibrator for measuring the temperature-coefficient with thick 

 threads. (4) A disk vibrator of brass, vibration taking place 

 about its axis of revolution. 



