MORTAR-MAKING. 905 



specific gravity may serve as a good guide or standard in 

 estimating the amount of mortar that should be mixed with 

 gravel for concrete ; for the difference between the weight of 

 a cubic foot of the aggregate when pressed together and 

 162^ lbs. will indicate the space to be filled. The difference 

 should be as accurately ascertained as possible, although it is 

 safer to have an overplus than too little of the necessary 

 hydraulic cementing material. 



One writer on the subject of " Foundations " remarks, 

 "The nature of concrete that should be used for foundations 

 depends on the nature of the soil it is to be laid in, the object 

 being in all cases to get as nearly as possible a homogeneous 

 bed under the structure. If the soil is wet, or the building is 

 of great weight or special character, the concrete should be 

 made of hydraulic lime and sand and broken stones, in about 

 the same proportions as would be used in rubble masonry, 

 that is to say, the lime should be about \, the sand about -f , 

 and the broken stones about 4- ' These, however, must be 

 considered only as average proportions. For medium 

 hydraulic lime and ordinary wet soils the proportion of 

 lime must be varied inversely as its quality is better or 

 worse, or as the circumstances are more or less important. 

 In such circumstances the concrete, if properly constituted 

 and laid, may be considered as a solid coherent mass, 

 capable of bearing without crushing the weight to the 

 square foot mentioned in recognised tables as the crushing 

 resistance of different kinds of concrete, a proper co-efficient 

 of safety being used. The bed of concrete must also be 

 thick enough not to break by transverse strain, but so as to 

 settle in one mass if the subsoil yields. These two considera- 

 tions will determine the area of the bed for the foundation. 



Considerable misapprehension exists as to the desirability of 

 keeping ground lime for any length of time before being used. 

 The amount of injury which lime in a finely powdered con- 

 dition receives from exposure arises from its avidity for 

 moisture. If, therefore, the air is excluded from it, and the 

 situation in which it is kept is dry, no injurious efiect of any 

 extent will arise. Smeaton found this, for he used Aberthaw 

 lime with great success in important engineering works after 

 it had been kept in casks for seven years. 



The use of Thiel lime in the construction ttf the works in 

 the Suez Canal is also confirmatory of the possibihty of using 

 lime after it has been some time reduced to powder. The 

 precaution, however, must be insisted on of keeping it only 



