468 Scientific Proceedings, Royal Diihlin Society. 



The pressure exerted by the peas is calculated as follows : — 

 Reading of balance x leverage ( pressure in lbs. per square inch exerted by 



Sectional area of piston ( peas. 



To the reading of the balance must be added a correction for the weight 

 of the lever, balance, and piston. When the balance is attached at six feet 

 from the fulcrum, this was found to be equal to ten pounds. 



To set up the apparatus the shackle bolt is slipped out and the piston 

 removed ; the seeds and sand are then carefully packed up to a level marked 

 on the cylinder. The piston and shackle bolt are replaced, and the spring 

 balance is attached. The water is then admitted gradually to the seeds from 

 below H, which is connected with a tubulated supply bottle arranged to 

 secure that the level of the water in the cylinder stands about an inch above 

 the piston. 



A considerable advantage is gained iu the use of sand and gravel in the 

 above apparatus ; this was clearly shown by three experiments set up with — 



(A) Beans packed carefully in sand over a large area of gravel. 



(B) Beans in sand but no gravel. 



(C) Beans alone. 



Iu (A) the pressures were higher and the rise more rapid than in (B). 

 This is due to the more ready access of water in (A) ; for in (B) the lowest 

 layer of sand becomes washed away, allowing the beans to come in contact 

 with the perforations of the gauze, choke them up, and thus greatly reduce 

 the water-supply. The much lower pressures in (C) demonstrated conclusively 

 the advantage gained by the use of sand. A control experiment was fitted 

 up in which sand but no seeds was put into the apparatus. On admitting 

 water no rise in the pressure took place, or rather could be recorded by the 

 balance, thus demonstrating that the greater rise of pressure in (A) and (B) 

 was not due to a pressure generated in the sand. 



In all experiments in which sand is used, it is important to admit the 

 water slowly, and from below, so as to secure the complete removal of air 

 from the spaces between the grains. In an interesting series of papers by 

 Jamin (10) an account is given of tiie forces generated in porous non-swelling 

 bodies by immersion in liquids : — 



" Je prends un bloc d'une matiere poreuse solide quelconque bien des- 

 sechee de craie, d'argile cuite, de pierre lithographique, de bois, etc., ou un 

 vase poreux de pile rempli d'une poudre fortement tassee, par exemple de 

 blane d'Espagne, d'oxyde de zinc, d'amidon et meme de terre dessechee. Je 

 oreuse dans la masse un trou cylindrique et j'y mastique un tube de mano- 

 metrique droit ferm^ parle haut, rempli d'air et contenant a sabase, un index 

 de mercure. II est clair que si la pression vieut a augmeuter dans la masse 



