HOGG. — FRICTION AND FORCE DUE To TRANSPIRATION. 



145 



other apparatus, depends entirely on the gaseous contents of the ves- 

 sel. There can be no force when these have all been removed. If there 

 is any vapor present, however, there will still remain some force when 

 the air has been pumped out. As we have seen, there are indications 

 that mercury vapor is present, but in small quantity, and at the lowest 

 pressure at which the transpiration apparatus was used the torsion 

 necessary to balance the force on the vane, were the contents of the 

 vessel the vapor alone, could not be more than 5°. This is readily 



FiGCRE 7. The curve shows the relation between pressure and force due 

 to gas action on a circidar vane over the range of pressure, p = 0.498 mm. to 

 p = 0.0093 mm. Tiie unit of pressure is 0.01 nun. The unit on the axis of ordi- 

 nates represents an angle of torsion of 10° given to the fibre supporting the vane. 

 This torsion is proportional to the force due to gas action on the vane. 



seen by considering the torsion at these small pressures to be propor- 

 tional to the pressure. 



If we confine our attention to the part of the curve from the origin 

 up to the maximum point, and use the larger values of the torsion 

 from which to determine the constants in Sutherland's equation, viz. : 



F 



Ap + B+ \Ip 



(IV) 



we get .1=0.00187, 7i = 0.0152, and 6'=25.G7. In the computa- 

 tions 0.005 mm. has been taken as the unit of pressure and 1° as the 



VOL. XLII. — 10 



