16 



PROCEEDINGS OP THE AMERICAN ACADEMY 



g/cs units. The pressure excess is the difference of pressures at 

 the two ends of the tube. 



TABLE I. — Mean Values of tj/10 9 for Marine Glue. 



In constructing the rate of change of viscosity with pressure, 

 I assumed that the whole thread transpired at the mean pressure at 

 the two ends of the steel tube; or since the pressure at the open end 

 is zero, at half the pressure excess. Furthermore, that 



V P = Vo 0- + ap) = r] (1 + £ a <\p). 



If therefore A r) be the increment of viscosity corresponding to 

 the pressure ^ Aj?, the final data of Table I. (rates) are at once 

 intelligible. 



In view of the occurrence of the arbitrary variable r] , it is hardly 

 probable that a would be independent of temperature. If reference 

 were made to some other pressure, viz. a' = (rj — rj ) / r} P) the val- 

 ues of a' approach each other in proportion as P is larger. Thus, 

 for P= 1000 atm., a' = .009 at 18°.3 and a' = .010 at 22°. 5. The 

 point of immediate importance here, however, is that in proportion 

 as the viscosity of a body increases with fall of temperature, its 

 isothermal rate of increase with pressure also increases; and that 

 the rate of increase in the latter instance is a coefficient as re- 

 markably large as a = .01 to .02. 



8. Viscosity and Temperature. Isopiestics. — Table I. also ena- 

 bles me to construct the approximate isopiestics for A p = 0, 500, 



* No flow perceptible after five hours, in case of a tube 10 cm. long and 1 cm. 

 in diameter. 



