548 Mr. G. A. Shakespear on an Interference- Method for 



vane-pillars are bolted to D 4 , and at their upper ends are screws 

 for the attachment of the cylindrical vanes. A dash-pot is 

 shown at h, fig. 7. 



The vaponr-jacket is in two parts, an inner and an outer. 

 The inner consists of two flat cylindrical boxes lying hori- 

 zontally, and vertically underneath one another, connected by 

 three vertical tubes. The top box is pierced by three tubu- 

 lures for the thermometers, the bottom one has four for 

 levelling-screws and weight-hook. There is a large inlet 

 for vapour at the top of the upper box, and an outlet at the 

 bottom of the lower box. The steel supporting-rod extends 

 through a tube in the centre of the upper box (on to a felt 

 washer), and D x is bolted firmly to it and against the lower 

 face of the upper box (see fig. 13). An annular trough 



round the edge of this box receives a flange from the upper 

 extremity of the outer part of the vapour-jacket, and a little 

 oil makes the joint draught-proof. 



The outer part of the jacket is simply a pair of concentric 

 cylinders of brass, with an inlet at top and outlet at bottom, 

 the annular space being closed in at both ends. The inner 

 cylinder projects at its base in a flange, which dips into a 

 trough on the outer edge of the lower flat box. There is a 

 double window of glass, at a suitable height, through the 

 outer jacket to allow of viewing the bands in the prism. 

 When in use the jacket is covered with felt. In putting in 

 fresh wires the outer jacket is raised, as in fig. 5, coarse 

 adjustments are made, and the bands got to a good clear 

 state, and then the outer jacket is lowered again into position. 



From the boiler a pipe passes to the inlets of the two parts 

 of the jacket, and from the outlets another pipe passes 

 through a condenser to the boiler, giving off below the con- 

 denser a small tube, which is connected with a manometer, 

 air-reservoir (a glass bottle of about 2 gallons capacity), and 

 water-pump. 



