202 Thickness, Cement and Materials of Wails. 



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 

 cr3^stallize, 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 theor}^, as it w^as warranted by ex- 

 perience, the common mortar of the masonry of the 

 Schuylkill bridge, wTtS composed of three parts sharp, 

 clean, coai'se 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- 

 qiiired. The proportion was less than a bushel to the 

 perch ; though no ver}^ exact attention was paid to this 

 circumstance. Even the interior mortar and grout, of 

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

 amined, were found perfectly indurated; after being 

 covered by water, for a few months only. Rich mortar y 

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

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

 water transportation, fro-cn Peters' s island, high up the 

 river, and far above all mai^hy and foul bottoms, or 

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

 It had every quality recommended by Anderson, and 



