Coming now to the subject of the cross section of beams, the rectangular section, if applied in the form of timber, could be made of such dimensions, by the adoption of one or other of the methods we have illustrated, as to bear great weights, such as merely by increasing the dimensions, as depth ab, breadth cd, fig. 474 (see succeeding paragraph); but it is obvious that this increase, as the weights they had to bear increased, would give such scantlings, so to say, that the mere weights of the beams would of themselves have no small influence on their breaking weight. To alter the form of the section would obviously, with such a material, involve no small manual labour. The case, however, was different with cast-iron, which could be run into sections or forms, such as scientific investigations or practical experience showed to be the best. The last, for a long time, was the girder; and the best section, and for long - and indeed yet - continued by many practical men to be the best, was that known as the " I" girder, in which the flanges at top and bottom are equal; and these and the central web at or about the same thickness. But the experiments, just above alluded to, conducted by Mr. Hodgkinson, showed that the strongest beam was that illustrated in a previous section, in which the sectional area of the top flange is one-sixth of that of the lower (see preceding illustration). This section is that now used in all cases where care is taken to secure the safest results, but, as just stated, many constructors, either indifferent to the indications of science or ignorant of what these have been, and are, still continue to use the old forms, and often, moreover, in the worst possible conditions. The calculations in connection with the improved section will be found further on.
59. Having thus described the strains by which various materials are influenced, the methods of ascertaining the amounts of these to which the parts of framing are subjected, and the direction in which these act, and alluded to the improved forms or sections of beams now generally approved of, we proceed to give a series of sentences containing a variety of practical details, which we trust will be useful to the student connected with the different departments of practical construction. We shall take the materials in what may be called their natural order.
60. In making the calculations respecting the pressure sustained by buildings and parts of buildings, it is necessary to know the weight per cubic foot of the stones, etc., most generally used throughout the country. The following list, which for obvious reasons is not complete, will, however, convey a fair amount of information useful to the student. We divide or classify the building stones of the kingdom thus - sandstones, first of England, second of Scotland; next the limestones, and of these, first the magnesium, and second the oolitic.
Taking first the English Sandstones, and of these the Abercame, from the quarries in Monmouthshire, we find the weight per cubic foot to be nearly 168 lbs.; of Ball Cross, Derbyshire, nearly 159 lbs.; of Barbadoes Quarry, in Monmouthshire, 146 ¾ lbs.; of Bevis's Quarry, Wiltshire, 111 lbs.; Bolton's Quarry, Yorkshire, 126 ¾ lbs. Bromley Hall, Yorkshire, 142 lbs. 3 oz.; Culverley, in Kent, 118 lbs.; Duffield Bank, Derbyshire, 132 lbs. 14 oz.; Gathally Moor, Yorkshire, 135 lbs. 13 oz.; Galton, Surrey, 103 lbs. 1 oz.; Heddon, in Northumberland, 130 lbs. 11 oz.; Hollington, Staffordshire, 133 lbs. 1 oz.; Lindley, Bed Quarry, Nottingham, 148 lbs. 10 oz.; Lindley White Quarry, 149 lbs. 9 oz.; Morley Moor, Derbyshire, 130 lb3. 8 oz.- Osmotherly Quarry, Yorkshire, 160 lbs.; Park Quarry, do., 160 lbs.; Park Quarry, Staffordshire, 124 lbs. 9 oz.; Park Spring, Yorkshire, 151 lbs. 1 oz.; Penshee Quarry, Durham, 134 lbs. 5 oz.; Shaw Lane Quarry, Derbyshire, 135 lbs. 15 oz.; Darley Dale Quarry, Derbyshire, 148 lbs. 3 oz.; Stanley Quarry, Shropshire, 146 lbs.; Slenton Quarry, Durham, 142 lbs. 8 oz.; Tulacre and Gweslyr Quarry, Flintshire, 150 lbs. 4 oz.; Victoria Quarry, Yorkshire, 145 lbs. 3 oz.; Viney Hall Quarry, Gloucester, 155 lbs. 11 oz.; Warwick Quarry, Yorkshire, 148 lbs. 10 oz.; Wheatwocd Quarry, Yorkshire, 143 lbs.; Whitby Company's Quarry, Arlasby, Yorkshire, 126 lbs. 11 oz.
The Scottish Sandstones - Auchray Quarry, Forfarshire, 158 lbs. 14 oz.; Binnie Quarry, Linlithgowshire, 140 lbs. 1 oz.; Cat Crag Quarry, do., 141 lbs. 11 oz.; Craigleith Quarry, Edinburgh, 145 lbs. 14 oz.; Crawbank, Linlithgowshire, 129 lbs. 2 oz.; Glammis Quarry, Forfarshire, 161 lbs. 2 oz.; Humbie Quarry, Linlithgowshire (white stone), 140 lbs. 3 oz.; do. (grey stone), 135 lbs. 13 oz.; Loch,Forfarshire, 159 lbs. 3 oz.; Lochee Quarry, 158 lbs. 11 oz.; Longannet, Perthshire, 131 lbs. 11 oz.; Munlochy, Ross-shire, 169 lbs. 9 oz.; Mylnefield Quarry, Perthshire, 160 lbs.; President Quarry, Dumbartonshire, 134 lbs. 5 oz.; Pigotdikes, Forfarshire, 162 lbs. 8 oz.
We now come to the limestones of the two groups, of which we take the magnesium first, - Bolsover, Derbyshire, 151 lbs. 11 oz.; Birdsworth, Yorkshire, 133 lbs. 10 oz.; Cadeby, Yorkshire, 126 lbs. 9 oz.; Huddllestone, Yorkshire, 137 lbs. 13 oz.; Park Hook Quarry, Yorkshire, 137 lbs. 3 oz.; Roche Abbey, Yorkshire, 127 lbs. 8 oz.; Shrawse do., Yorkshire, do. do. Taking next the oolitic limestones, we come first to those of the limestone quarry, Lincolnshire, a cubic foot of which weighs 139 lbs. 4 oz.; Bath Lodge Hill Quary, Somerset, 116 lbs.; Bath Baynton Quarry, Wiltshire, 123 lbs.; Haydon, Lincolnshire, 133 lbs. 7 oz.; Kelton, Rutlandshire, 128 lbs. 5 oz.; Portland (Vern St. Quarry), 134 lbs. 10 oz.; Portland (Waycroft Quarries), 135 lbs. 8 oz.; Portland (Grove Quarry), 145 lbs. 9 oz.
61. Although cast-iron, for the purposes of columns to support heavy weights, has greatly, indeed almost wholly, superseded the use of stone, in some cases, however, this latter material is still used. When so, the length should never be greater than twelve times the diameter of the column at the base; nor should the load which the column has to support exceed one-tenth of the estimated weight or pressure which it takes to crush or disintegrate the particles of the stone of which the column is built. According to the best experiments which have been made, it appears that granite can stand a pressure of 500 tons per square foot before fracture is caused; marble, 400; sandstone, 350; Craigh Leaf freestone, 200; magnesian limestone, 100; brick of the best quality, 70; of ordinary, 40; while fire-brick is as high as 75. A brickwork column, set with lime joints, takes a pressure of 20 tons per square foot before it crushes or breaks; if the bricks are set with cement, the pressure is increased to 30 tons; while a rubble masonry column, with lime setting, is of the same strength as one of brickwork with lime joints, namely, 20. Of the two cements chiefly used, the following gives the cohesive strength when used in setting bricks. "Roman pure;" 467 lbs., one part cement and one sand, 420½ lbs. "Portland pure," 152 lbs.; one part cement, three of sand, 420 lbs. Of the weights, etc., of soils, a cartload averages 2½ tons. A cubic yard is otherwise and generally designated a "load." An ordinary sized wheelbarrow is estimated to hold a tenth of a cubic yard, or in other words, ten wheelbarrow loads make an ordinary load of a cubic yard. Soils, when dug take up greater space than when in situ; thus ordinary soil increases as much as 25 per cent, in bulk, so that three wheelbarrow loads in its natural condition will form four. Sand and gravel increase one-twelfth, chalk a third. The absorptive power of soils vary very considerably; thus 100 lbs. of pure sand will absorb one-fourth of its weight of water; pure clay, 70; chalk, 45. White pure clay is the most cohesive of ordinary soils, and is therefore taken as the standard, or say 100, ordinary soil is 33, sandy soil 57, loamy clay 68; while pure sand, which is wholly, or nearly, destitute of cohesive powers, is taken at zero. Soils when thrown up in a loose condition have a tendency to slide down, and assume the horizontal position; there is, however, a limit to this, and there is an angle at which they will remain quiescent, this technically is called the "angle of repose," and is for various materials as follows: for ordinary soil the angle of repose is 28°; gravel, 40°; dry sand, 38°; sand in its normal condition, 22°; un-drained clay, 16°; drained clay, 45°.