Vol. 51.] 



HOW OF A VISCOUS FLUID. 



363 



level at the closed end ; the line 115 mm. distant lay immediately 

 over the proximal face of the now buried barrier. 



The object of the experiment will be seen from the following 

 simple reasoning. 



Since in fig. 2 the downward pressure due to gravity is greater 



Fig. 2. 



over that side of the mass of pitch which lies under A than over 

 that under B, a movement and transfer of matter will take place 

 from A towards B, till the two sides are in equilibrium. The 

 transference might be partly accomplished by a superficial sliding 

 down the slope AB, or as well by a general flow in the direction of 

 the arrows. 



But supposing the general flow to take place, one may ask 

 whether it will be confined to the material which lies above the 

 line a v b v or extend throughout the whole mass. This, indeed, is 

 precisely the question to which the experiment was expected to 

 find an answer. 



One of the first phenomena observed was a marked sinking of 

 the surface near the upper end of the ' glacier,' which made itself 

 obvious simultaneously with the downward or forward curving of 

 the transverse lines that had been ruled across the surface ; a curva- 

 ture, needless to add, due to the more rapid rate of flow near the 

 centre of the trough. The sagging near the source was accom- 

 panied by a bulging immediately above the barrier, but this was 

 less marked because the rising material to which it was due was 

 comparatively rapidly carried away as it cascaded over the preci- 

 pitous distal face of the barrier. 



The movement of the transverse lines ruled on the surface was 

 measured from time to time, and at irregular intervals, usually of 

 a little over a week, fresh pitch was added to restore the original 

 slope, which, owing to the continual sinking near the upper end, 

 was diminished daily as much as one degree. A layer of powdered 

 pigment was usually dusted on to the old surface before adding 

 fresh pitch. 



The experiment was started on December 27th, 1894, and brought 

 to an end on March 24th, 1895, when the ' glacier ' was split open 

 longitudinally down the middle. During the eighty-seven days of 

 its existence it had flowed at a fairly uniform rate, the average 



