72 



Roiad Society. 



T. 



50 



20 



10 



5 



4 



3 



2 



1 



100 



^ 8 



3 

 4 



Tn the above experiments it will be observed that the distance 

 between the pith disk and the s\vinging glass disk was constant, 

 viz. 100 milhins. We have made several series of experiments, 

 wath the disks at various distances, in atmospheres of various 

 tensions. The means of several of these series are graphically 

 represented in the following diagram. Tb.e ordinates represent 

 the distance in millimetres through which the index moved in a 

 period of 15 seconds; the abscissas represent the distance in 

 millimetres between the disks. 



Our expectation of being able to ascertain the thickness of the 

 Crookes's laj^er has not been entirely fulfilled with the apparatus r s 

 at present constructed, owing to the presence of the exceedingly- 

 feeble force that is represented by the horizontal portions of the 

 diagram. We have found this feeble force nearly constant at 



