202 Thickness^ Cement and Materials of Walls. 



sand: and next to this pit sand; on these accounts. But 

 pit sand is generally smoother, smaller and less angu- 

 lar; and more mixed with loam, or earth. Sea sand is 

 more subject to these objections as to its form ; if it were 

 otherwise proper ; and those who build near sea coasts, 

 should use pit sand in preference ; as it is rougher, and 

 has no saline mixture. More lime, than will plentifully 

 coat the surface, is worse than unnecessary ; as it cannot 

 crystallize beyond a certain point ; and the extra quan- 

 tity having no sand, or substance, around which it can 

 crystallize, repels, or prevents, the approaches of the 

 grains of coated sand; so as to obstruct their adhesion, 

 and forming, by general crystallization, a solid mass. 

 With a view to this theory, as it was warranted by ex- 

 perience, the common mortar of the masonry of the 

 Schuylkill bridge, was composed of three paits sharp, 

 clean, coarse sand, and one part lime. The sand was 

 thrown into a bed of thin wash, of slacked lime, and 

 agitated till every grain was coated; and then, additions 

 of sand were made, till the proper consistency was ac- 

 quired. The proportion was less than a bushel to the 

 perch ; though no very exact attention was paid to this 

 circumstance. Even the interior mortar and grout, of 

 the thickest walls and piers, so for as they could be ex- 

 amined, were found perfectly indurated; after being 

 covered by water, for a few months on!}'-. Rich mortar, 

 is therefore one cause of the loose texture of thick walls. 

 The sand, for the bridge masonry, was obtained, by 

 water transportation, from Peters'' s island, high up the 

 river, and far above all marshy and foul bottoms, or 

 shores. Near 12000 cart loads of this sand were used. 

 Tt had every quality recommended by Anderson, and 



