82 



have to be 840 ft. high before the bottom courses would 

 commence to crush from the weight of the brickwork 

 above. 



Average sandstones at 145 Ib. per cu. ft. would require 

 a column 5950 ft. high to crush the bottom stones; an 

 average granite at 165 Ib. per cu. ft. would require a 

 column 10,470 ft. high. The Merchants' shot-tower at 

 Baltimore is 246 ft. high, and its base sustains a pres- 

 sure of six tons and a half (of 2,240 Ib.) per sq. ft. The 

 base of the granite pier of Saltash Bridge (by Brunei) of 

 solid masonry to the height of 96 ft., and supporting the 

 ends of two iron spans of 455 ft. each, sustains 9 tons 

 and a half per sq. ft. 



WEIGHTS PER SQ. FT. OF ROOF SURFACE. (Kldder). 



Shingles, common, 2% Ib. ; 18 in., 3 Ib. 



Slates, 3-16 in. thick, 7% Ib.; % in. thick, 9.6 Ib. 



Plain tiles or clay shingles, 11 to 14 Ib. 



Roman tiles, old style, 2 parts, 12 Ib.; new style, 1 part, 

 8 Ib. 



Spanish tiles, old style, 2 parts, 19 Ib.; new style, 1 

 part, 8 Ib. 



Improved Oriental tiles, 11 Ib. 



Ludowici tile, 8 Ib. 



(For tiles laid in mortar, add 10 Ib. per sq. ft.) 



Copper roofing, sheets, iy a Ib.; tiles, 1% Ib. 



Tin roofing, sheets or shingles, including 1 thickness of 

 felt, 1 Ib. 



Corrugated iron, painted or galvanized, No. 26, 1 Ib.; 

 No. 24, 1.3 Ib.; No. 22, 1.6 Ib. ; No. 20, 1.9 Ib.; No. 18, 

 2.6 Ib.; No. 16, 3.3 Ib. 



Standing seam steel roofing, 1 Ib. 



Five ply felt and gravel roof, 6 Ib. 



Four ply felt and gravel roof, 5V Ib. 



Three ply ready roofing, (elaterite, ruberoid, asphalt, 

 etc.), 0.6 to 1 Ib. 



Skylights with galvanized iron frame, % in. glass, 4% 

 Ib.; 5-16 in., 5 Ib.; % in., 6 Ib. 



Sheathing, 1 in. thick, 3 Ib. per sq. ft. for white pine, 

 spruce or hemlock; 4 Ib. for yellow or pitch pine. 



