858 Transactions of the American Institute. 



bers are placed apart, and the spaces made equal to the width of 

 the timbers, and filled with concrete* masonry. 



The concrete becomes hard enough to take the place of the timber 

 before the latter decays. Some parts of a structure frequently 

 bring greater weight upon that part of the foundation directly 

 below it, than others, either because the walls are heavier or are 

 loaded heavier, or by the form of the walls themselves, and care 

 must be taken either to distribute this excess of weight over more 

 of the surface, or to strengthen the foundation at such places. 



The former is done by means of inverted arches, and still better, 

 by direct and inverted arches. 



When the circumstances of the case forbid the use of wood in the 

 foundations, and it is desired to extend the base be^'ond the exterior 

 lines of the masonr3% or to equalize the pressure over the whole 

 surface, a thick course of concrete masonry is spread over the 

 bottom. As the cohesive strength of concrete, even when made in 

 the best manner, is much less than that of timber, it must have a 

 much greater depth to allow the same extent of expansion to the 

 foundation. 



Where hydraulic cement is expensive, the same object can be 

 attained by the use of a heavy bed of small broken stone, and in 

 some cases, gravel may be substituted with economy, and accom- 

 plish the desired end. 



In these cases, and particularly with broken stone, the angular 

 projections form a sort of mechanical an-angement, which distributes 

 the weight brought upon each piece, to several of those beneath 

 it, and by this means, the weight is distributed equally over the 

 bottom, if the ma-ss of stone is of sufficient thickness. The same 

 result is obtained with concrete, with the additional advantage, that 

 the angular projections of the stone are supported by the hydraulic 

 mortar around it, which also binds the fragments together, and 

 produces the result desired with less depth of material.f 



A common practice is to form the first course of foundation 

 masonry of large thin stone, which answers a good purpose, if 

 these stone are strong enough to sustain the weight without breaking. 



In the preceding remarks, it will be observed that the leading 



*Id Europe, the term concrete is applied to gravel mixed with quicklime mortar, and 

 betoD, to small broken stone, mixed with hydraulic cement mortar. American engineora 

 rarely use the former, and apply the term concrete to the latter. 



\ There is a popular expression that "cement becomes as hard as stone." The best of 

 it has about one-fourth the reBistanoe against crushing, of sandstone, and one-sixth of that 

 of granite. 



