1910] on Electriral and otlipr Propprties of Sand. 7-4!) 



Regarding the cell as the section of a tube, I = 2r and A = r tan 

 35°. Therefore, substituting these values in (1), we have 



H = ^^ = ^^^ . 



2 tan 35° 1-4 



The ratio of the force applied vertically, to that of the lateral 

 thrust, is thus equal to twice the tangent of the angle which tlie slope 

 of a cone makes with the horizontal, viz. 1 • 4. 



For instance, if the vertical force due to a weight placed on the 

 sand is 100 lb., the lateral pressure will amount to about 71 lb. 

 A piston resting upon a column of sand only a few diameters high, 

 contained in a sirong tube closed at its lower end by merely a thin 

 membrane, is capable therefore of sustaining very heavy loads. 



In order to demonstrate this on a moderate scale I have arranged 

 a sort of gallows, through the projecting arm of which a flanged 

 brass tube is inserted vertically. This tube is 0*5 inch in diameter, 

 and closed at its lower extremity with a piece of cigarette paper held 

 in position by an indiarubber band. A small quantity of sand is 

 tipped into the tube from above — enough to fill it to a height of 

 3 inches. The column within will therefore measure 6 diameters. 

 The tube is then well tapped to ensure normal piling of the grains, 

 and a loosely fitting iron plunger is inserted so as to rest upon the 

 sand. Attached to the plunger is a cross-piece carrying a ring at 

 each end which may be grasped with the hands. My assistant (who 

 weighs about 11 stone) thus suspends himself safely, his weight being 

 supported by the small sand column. If the piece of cigarette paper 

 is now removed he is let down with an unpleasant jerk. 



Some idea of the close arrangement of the particles may be 

 gathered by noticing that a long column of sand, moving downward 

 within such a tube, will produce a vacuum above it sufficient to lift 

 water to a height of about 6 feet. (Experimentally shown.) 



These experiments, upon loaded sand columns, clearly prove there- 

 fore how it is that the " head " is destroyed, and explain why the 

 powder issues from an orifice at a uniform rate. 



Lord Rayleigh applied this principle to a very interesting device 

 which he used here some years ago for the purpose of slowly rotating 

 a smoked disc. A weight stood upon a sand column contained in a 

 glass tube. Its downward motion as the column lowered, due to 

 escape of powder from a nozzle at the end, served to operate a train 

 of wheels. The question arises, however, as to whether such a motion 

 is quite uniform. In other words, does the sand move regularly in 

 the tube ? Experiments indicate that it is very difficult to obtain 

 an absolutely uniform motion by this means. Friction appears to be 

 the controlling factor. A tube oiled upon its inner surface is now 



