Viscosity encountered on passing from Fluid to Solid. 339 



gauge, g g, held in place by a 

 layer of paraffin, h, shows the 

 observer to what degree the 

 vacuum is maintained in the 

 lapse of time. A number of 

 tubes, a be, may be inserted 

 side by side, and it is always 

 necessary to coat the stopper 

 with cement. 



5. The transpiration-equa- 

 tion due to Poiseuille, theoreti- 

 cally corrected by Hagenbach*, 

 has the form 



n 



ttIV 



Svl 



t— 



m 



V^irl t' 



(!) 



Fig. 1. — Apparatus for the Secular 

 Transpiration of Viscous Liquids. 





where tj is the absolute visco- 

 sity of the liquid, v the volume, 

 and m the mass transpiring 

 through the capillary length I 

 and radius r, in the time t. 

 In the present case of exces- 

 sively slow transpiration the 

 correction may be neglected. 

 P is the pressure-excess, in 

 dynes per square centimetre. 

 All magnitudes are to be ex- 

 pressed in C.G.S. units. 



In the above form of appa- 

 ratus (fig. 1) I is not constant, 

 and v is measured in terms of 

 the increase of length of the 

 capillary thread a b (fig. 1). 

 Hence the differential equation 

 corresponding to (1), since 

 dv = irr 2 dl, is 



= IV eft 

 V SI dC 



• (2) 



Integrating between and t, and between l x and l 2 , 



v = TrHm^-^) (3) 



Here Z a is the original length (£=0), and l 2 the final length 



* Pcgg. Ann. cix. p. 358 (18C0). This reference is incidental, or I 

 should have to refer to Foisson, Navier, Stokes, Stefan, Helmholtz, and 

 others. 



