Hollow columns fail principally by crushing, provided the length does not exceed 25 diameters; indeed, the length does not appear to affect the strength much till it exceeds 50 diameters.

The comparative strength of different forms and of different thicknesses will appear so distinctly from the experiments below, made by Mr. Hodg-kinson, that no difficulty will be found in ascertaining the strength due to any size or form of column that may be required.

Square Columns of Plate Iron Rivetted

 Columns 10ft. 0 in. long. Size. Thickness. Proportion of Thickness to Width. Proportion ofLength to Width. Break'g weightTons per sq. in.of section. 4 in. X 4 in. •03 1/133 30 to 1 4.9 " •06 1/66 " 8.6 " •1 l/40 " 10. " •2 1/20 " 12. 8 in. X 8 in. •06 1133 15 to 1 6. " .14 1/60 " 9. " •22 1/36 " 11.5 " •25 1/32 " 12. Column 8 feet 0 inches long. 18 X 18 .5 1/30 practically 5.4 to 1 13.6 Column 10 feet 0 inches long, with Cells. 8 in. X 8 in. •06 1/66- of width of cells 15 to 1 8.6

To find the strength of any Hollow Wrought Iron Column.

Sec. area, sq. ins x Tons per inch, corresponding to the proportions of length and thickness to width as per tables =

Breaking weight, tons.

Columns of Oblong Section.

The strength of these may be ascertained by the same rule as that of square columns. The smallest width being taken in calculating the proportion of height to width, while the longest side must be taken into consideration in calculating the proportion of thickness to width.

Column 10 feet 0 inches long.

 Size. Thickness. Proportion ofThickness to greatest Width. Proportion ofLength to leastWidth. Actual Breaking Weight Tons per sq. in. of Section. 8 in. X4in. •06 1/333 30 to 1 6.78

Round Columns of Plate Iron Riveited.

 Columns 10 ft. 0 in. long. Same Columns Reduced in Length. Diameter. Thickness. Proportion of thickness to Diameter. Proportion of length to Diameter. Breaking Weight."Tons per sq. inch. Breaking Weights. Tons per square inch. 5 ft. 0 in. long. 2 ft. 6 in. long. 1 ½ •1 1/15 80 to 1 6.-5 139 5.8 2 •1 1/20 60 to 1 10.35 14.8 16.5 2½ •1 I 48 to l 13.3 156 16.3 2½ •24 l/11 48 to 1 9.6 156 16. 2½ •21 1/12 48 to 1 9.9 13. 17. 3 •15 1/26 40 to l 12.36 13. 16.5 4 •15 1/26 30 to 1 12.34 13. 6 •1 1/60 20 to l 15. 17. 186 6 •13 1/43 20 to l 18.6

It would seem from this that a thickness of 1-48, or 1-4 inch in thickness for every foot in diameter is a good proportion for this kind of column.

It will be seen from these experiments, that it is the proportion of thickness to the width of cell which regulates the strength within certain limits of height.

And that a thickness of 1-30 or 1-8 inch for every 4 inches in width will give the highest result practicable for square columns.