1880.] Dr. J. Burdon-Sanderson. On a New Rheotome. 385 



The fixed circular plate above described is surrounded by a moveable 

 annular plate of the same material and thickness, which also has eight 

 excavations or pools, all of which can be brought into communication 



Fig. 1. 



by means of an annular trough of mercury by which, they are sur- 

 rounded. To the outer edge of the annular plate is fixed a graduated 

 circle of brass, which revolves with it. Against this circle works a 

 tangent screw, each turn of which corresponds to a single division of 

 the graduation ; this screw is furnished with a divided milled head 

 and can be thrown out of gear when necessary by the auxiliary screw, 

 in a manner which will be readily understood from the drawing. The 

 vertical axis carries at its upper end a pulley, by means of which it is 

 connected with a motor, and can be made to revolve at any desired 

 rate. Above the pulley is a platinum wire, the end of which is bent 

 downwards so as to dip into an annular pool of mercury, which is in 

 metallic connexion with the horizontal bar, and thereby with the iron 

 plate and the binding screw B'. The vertical axis also carries three 

 arms, of which one is longer than the other two ; the longer arm carries 

 at its end the vertical screw g ; this screw is prolonged at its 

 lower end by an amalgamated gold wire, the end of which dips into 

 the mercury contained in the external pool, as the axis rotates. This 

 arm is insulated at c, but is connected with the axis, and thereby 

 through the annular pool above described with the binding screw B'. 

 As the external pools are connected through the arm h with the bind- 

 ing screw B, B and B' are in communication whenever the screw g 

 is in contact with the mercury in any of the pools. Each pool is fur- 

 nished with a vulcanite stop, by which it is thrown out of communi- 

 cation with the external pool. In the drawing all are thus out of 

 communication excepting one. 



The two shorter arms are in metallic connexion with each other, 

 but are insulated from the axis by a collar of vulcanite d. One of 

 them carries a screw e, which like g ends in a gold wire, of which the 

 tip is amalgamated ; the other carries a similar screw /, and is so 

 constructed that the screw/ can be moved radially (i.e., in a vertical 



