\ 



1886.] on Experiments showing Dilatancy. 359 



is put on tliG bag, but it does not change its sliai^c at all ; tlie shape 

 cannot alter without enlarging the interstices, these cannot enlarge 

 without drawing more water, and this is prevented. To show that 

 there is an effort to enlarge going on, it is only necessary to open a 

 communication with a pressure-gauge, as in the exj)eriment with air. 

 The mercury rises on the side of the bag, showing when the pinch is 

 hardest (about 200 lbs. on the planes) that the pressure in the bag is 

 less by 27 inches of mercury than the pressure of the atmosphere ; a 

 little more squeezing and there is a vacuum in the bag. Without a 

 knowledge of the property of dilatancy such a method of producing 

 a vacuum would sound somewhat paradoxical. Opening the neck to 

 allow the entrance of water, the bag at once yields to a slight pressure, 

 changing shape, but this change at once stops when the sujDply is cut 

 off, preventing further dilation. 



In these experiments neither the thickness of the bag nor the 

 character of the fluid has anything to do with the dilation of the con- 

 tents considered as forming an interior group of a continuous medium, 

 the bag merely controlling the outside members as they would be 

 controlled by surrounding grains, and the fluid merely measuring or 

 limiting the volume of the interstices. 



It has, however, been absence of such control of the outside grains 

 and such means of measuring the volume of the interstices that has 

 lire vented the dilatancy revealing itself as a general mechanical 

 property of granular material, as a mechanical property, because 

 dilatancy has long been known to those who buy and sell corn. It 

 is seldom left for the philosopher to discover anything which has a 

 direct influence on pecuniary interests ; and when corn was bought and 

 sold by measure it was in the interest of the vendor to make the 

 interstices as large as possible, and of the vendee to make them as 

 small ; of the vendor to make the corn lie as lightly as possible, and 

 of the vendee to get it as dense as possible. These interests are 

 obvious ; but the methods of getting corn dense and light are para- 

 doxical when compared with the methods for other material. If we 

 want to get any elastic material light we shake it up, as a pillow or a 

 feather bed, or a basket of dried fruit; to get these dense we 

 squeeze them into the measure. With corn it is the reverse ; it is no 

 good squeezing it to get it dense ; if we try to press it into the measure 

 we make it light — to get it dense we must shake it — which owing to 

 the surface of the measure being free, causes a rearrangement in 

 which the grains take the closest order. 



At the present day the measure for corn has been replaced by the 

 scales, but years ago corn was bought and sold by measure only, 

 and measuring was then an art which is still preserved. It is under- 

 stood that the corn is to be measured light, and the method 

 employed is now seen to have made use of the property of dilatancy. 

 The measure is filled over full and the toj) struck with a round pin 

 called the strake or strickle. The universal art is to put the strake end 

 on into the measure before commencing to fill it. Then when heaped 



