1846.] ADDITION TO MB. CHKlSTIE's OBSERVATION. 167 



though uninterruptedly, and the level of the water in the vertical 

 shaft slowly descends, indicating the diminution of internal 

 pressure. If it were not for the capillarity of the ducts, it is plain 

 that no effective hydrostatic pressure would be developed at all ; 

 the flow being equal to the supply, no part of the vis viva would 

 be expended in producing internal pressures. With this con- 

 cluding observation I commit the Plastic or Viscous Theory of 

 Glaciers to the impartial judgment of those qualified to decide 

 on its merits in explaining facts, and on the variety of difficult 

 and complicated considerations which opposed and still oppose 

 themselves to a complete development of it. 



Edinburgh, Jan. 10, 1846.* 



ADDITION TO THE FOOTNOTE at page 161 of the preceding 

 Paper ON THE PLASTICITY OF ICE ON A SMALL SCALE. 



[I take this opportunity of recording, that soon after the 

 publication of this paper (in 1846) I made some experiments 

 connected with Mr. Christie's ingenious observation. The 

 freezing of water was performed in strong glass vessels, so that 

 the manner of protrusion of the ice could be better examined. 

 It was conceivable that the expulsion of the ice in the cast-iron 

 shell through the cylindrical opening was facilitated by the 

 internal pressure punching out, as it were, successive cylinders 

 of ice from the spherical shells of ice successively formed, or 

 that the extrusion was accomplished by a series of cylindric 

 fractures, and not by the general moulding of the plastic ice 

 from a wider through a narrower outlet. To show that the 

 latter and not the former assumption was correct, I took some 

 greasy matter of a bright red colour, and introducing it by the 

 finger through the aperture of the strong glass vessel, I anointed 

 with it the inside of the glass all round the internal orifice or 

 throat of the aperture, keeping, however, the walls of the aperture 



* [Printed by mistake 1845 in the Phil. Trans.] 



