360 Professor Osborne Beynolds [Feb. 12, 



full, to pull the strake gently out and strike the top ; if now the 

 measure be shaken it will be seen that it is only nine-tenths full. 



Sand presents many striking phenomena well known but not 

 hitherto explained, which are now seen to be simply evidence of 

 dilatancy. 



Every one who walks on the strand must have been painfully 

 struck with the difference in the firmness and softness of the sand at 

 different times ; letting alone when it is quite dry and loose. At 

 one time it will be so firm and hard that you may walk with high 

 heels without leaving a footprint ; while at others, although the sand is 

 not dry, one sinks in so as to make walking painful. Had you noticed 

 you would have found that the sand is firm as the tide falls, and 

 becomes soft again after it has been left dr}^ for some hours. The 

 reason for this difference is exactly the same as that of the closed 

 bags with water and air in the interstices of the sand. The tide 

 leaves the sand, though apparently dry on the surface, with all its 

 interstices perfectly full of water which is kept up to the surface of 

 the sand by capillary attraction ; at the same time the water is perco- 

 lating through the sand from the sands above where the capillary 

 action is not sufiicient to hold the water. "When the foot falls on this 

 water-saturated sand, it tends to change its shape, but it cannot do 

 this without enlarging the interstices — without drawing in more 

 water. This is a work of time, so that the foot is gone again before 

 the sand has yielded. If you stand still, you will find that your feet 

 sink more or less, and that when you move, the sand becomes wet all 

 round the space you stood on, which is the excess of water you have 

 drawn in, set free by the sand regaining its densest form. 



One phenomenon attending walking on firm sand is very striking ; as 

 the foot falls, the sand all round appears to shoot white or dry 

 momentarily, soon becoming dark again. This is the suction into 

 the enlarging interstices below the foot, which for the moment 

 depresses the capillary surface of the water below that of the sand. 



After the tide has left the sand for a sufiicient time, the greater 

 part of the water has run out of the interstices, leaving them full of 

 air, which by expanding allows the interstices to enlarge, and the 

 foot to sink in far enough to make walking unpleasant. 



If we walk on sand under water, it is always more or less soft, for 

 the interstices can enlarge, drawing in water from above. 



The firmness of the sand is thus seen to be due to the interstices 

 being full of water, and to the capillary action or surface tension of 

 the water at the surface of the sand. Tliis capillary action will hold 

 the water up in the sand for some inches or feet, according to the 

 fineness of the sand. This is shown by a somewhat striking experi- 

 ment. If sand running in a stream from a small hole in the bottom 

 of a vessel, as in an hour-glass, fall into a vessel containing a slight 

 depth of water, the sand at first forms an island, which rises above 

 the water. The sand which then falls on the top of this island is dry 

 as it falls, but capillary action draws up the water which fills the 



