OF ARTS AND SCIENCES. 



17 



1000, and 2000 atm. These loci are distinctly curved, showing 

 that viscosity decreases at an accelerated rate with rise of tem- 

 perature. 



9. Conditions of Constant Viscosity. Isometrics. — In order, how- 

 ever, to compare the relative effects of temperature and pressure, it 

 is necessary to map out the lines of equal viscosity for marine glue. 

 I endeavored to do this hy constructing Table I. graphically; thus 

 I attained the data of Table II., where Ap is the pressure excess 

 corresponding to the temperature 0, for the case in which the vis- 

 cosity r) has the value given at the head of the table. 



TABLE II. — Isometrics as regards Viscosity for Marine Glue. 



There are thus given a family of lines of decreasing curvature. 

 Supposing them to be eventually more nearly linear I have con- 

 structed fi = k.6 l\ Ap (where A 6 and ^ A p are the correspond- 

 ing increments of temperature and pressure for constant viscosity) 

 for the region between Aj) = 1000 and 2000 atm. See Table II. 

 These values, crude as they are, have an important geological bear- 

 ing on the fluidity of magmas. I infer that in a field of high pres- 

 sure as small a rise of temperature as 1° C. will quite wipe out 

 the increment of viscosity due to a pressure as large as 400 atmos- 

 pheres. 



10. Isothermal Flow through Tubes. — Let there be given a tube 

 of length I and radius p. Let t] = -q (1 + a p) be the viscosity of 

 the viscous liquid forced through it by the pressure excess A p = 2 p 

 (so that there is no pressure at one end of the tube) and at the con- 

 stant temperature 6. The length A of the cylinder of fluid issuing 

 per unit of time (t) will be 



vol. xxvn. (n. s. xix.) 2 



