448 



Mr. W. Crookes. 



[Feb. 17, 



the axis of the tube. Sockets are firmly fixed to a solid support, so 

 that when the tube and bulb are clamped between them they are only 

 able to move around the vertical axis. The glass fibre being only con- 

 nected with the tube at the top, rotating the tube on its axis com- 

 municates torsion to the fibre, and sets the mica plate swinging on the 

 same axis without giving it any pendulous movement. The diameter 

 of the fibre is about 0*001 inch. The viscosity apparatus is connected 

 to the pump by a flexible glass spiral, so as to allow the apparatus to 

 rotate on the pivots and at the same time to be connected to the pump 

 altogether with sealed glass joints. An arm working between metal 

 stops, limits the rotation to the small angle only which is necessary. 



The torsional movement given to the mica plate, by the light of the 

 candle shining on it or by the rotation of " the bulb and tube on its axis 

 by the movement of the arm between the stops, is measured by a beam 

 of light from a lamp, reflected from a mirror to a graduated scale. 



The pump employed has already been described. The measuring 

 apparatus is similar to that described by Professor McLeod* before 

 the Physical Society, June 13fch, 1874. As it contains several improve- 

 ments shown by experience to be necessary when working at very high 

 vacua, a detailed description is given in the paper. 



When taking an observation the arm is moved over to the stop, 

 and in a few seconds allowed to return to its original position by the 

 action of a spring. This movement rotates the viscosity apparatus 

 through a small angle, and sets the mica plate vibrating, the reflected 

 line of light traversing from one side of the scale to the other in arcs 

 of diminishing amplitude till it finally settles down once more at 

 zero. 



The observer watching the moving index of light records the scale 

 number reached at the extremity of each arc. The numbers being 

 alternately on one and the other side of zero are added two by two 

 together, to get the value of each oscillation. The logarithms of 

 these values are then found and their differences taken ; the mean of 

 these differences is logarithmic decrement per swing of the arc of 

 oscillation. For the sake of brevity this is called the log dec. 



A very large number of experiments have been made on the vis- 

 cosity of air and other gases. Observations have been taken at as 

 high an exhaustion as 0'02 M, but at these high points they are not 

 sufficiently concordant to be trustworthy. The pump will exhaust to 

 this point without difficulty if a few precautions are taken, but at this 

 low pressure the means of measuring fail in accuracy. 



The precautions which experience shows to be necessary when 

 exhausting to the highest points are fully described in the paper. 



* " Phil. Mag.," vol. xlviii, p. 110, August, 1874. 



