by Small Plates exposed to a Stream of Fluid. 181 



In view of the smallness of this quantity, it is not surprising 

 that the influence of linear scale should fail to manifest 

 itself. 



In virtue of the more complete symmetry realisable when 

 the plates to be compared are not merely equal in area but 

 also similar in shape, this method would be specially advan- 

 tageous for the investigation of the possible influence of 

 thickness and of the smoothness of the surfaces. 



When the areas to be compared are unequal, so that their 

 centres need to be at different distances from the axis, the 

 resistance balance of the auxiliary parts demands special 

 attention. I have experimented upon circular disks whose 

 areas are as 2 : 1. When there was but one smaller disk 

 (6 cm. in diameter) the arms of the lever have to be also 

 as 2 : 1 (fig. 1). In another experiment two small disks 



Kff. 1. 



(each 4 cm. in diameter) were balanced against a larger one 

 of equal total area (fig. 2). Probably this arrangement is 



Fig. 2. 



the better. In neither case was any difference of mean 

 pressures detected. 



In the figures AA represents the needle, B and C the 

 large and small disks respectively, D the extra attachments 

 needed for the resistance balance of the auxiliary paits. 



