Glazed brick |8.8 × 4.4 × 3.3 | 69 | 166 | -- |Frog. „ „ |8.9 × 4.4 × 2.9 | 166 | 174 | -- | ---------------+----------------+-----------+----------+--------+------------ Table No. III. is specially instructive as indicating the relative strength of several well-known bricks, the experiments being carried out solely for scientific purposes. Yet the figures must not be taken too seriously. Glancing at those relating to “London Stocks,” we find the strength varied from 103 tons per square foot to 181 tons. But more recent experiments made by Professor Unwin[19] on some London Stocks from Sittingbourne, in Kent, shewed that with four samples one crushed at 60.76 tons per square foot and another gave out 94.6 tons, the mean strength of the four yielding 84.27 tons per square foot. With such heterogeneous materials as London Stocks, we ought not to be surprised at these results, but they form a striking commentary on the value of general statements concerning the strength of bricks of varied character going by the same name in the market. When we consider the strength of homogeneous bricks, and especially where these latter are made of thick marine clays, or where the relative proportions of earths employed are carefully attended to in the raw material, the results appear to be more generally applicable--as far as they go. With ordinary Gault bricks we find a range in strength from 145 tons to 173 tons per square foot; but Professor Unwin,[20] in his more recent experiments, finds that of four Gault bricks, one reached as high as 197.6 tons per square foot, and he gives 182.2 tons as the average strength. To shew the absurdity of alluding to the strength of “blue Staffordshire” bricks, without also giving the precise locale of the samples dealt with, the reader is requested to refer to Table III., where the figures indicate a range from 275 tons to 464 tons per square foot, and to compare them with the results on Staffordshire bricks as stated in Table II., where we find a range from 651 tons to 1,064.2 tons per square foot. Of what value can a single formula be which gives the strength of Staffordshire bricks as a whole as based on such widely divergent figures as these? Professor Unwin, in his recent series of experiments alluded to, finds that with four Staffordshire blue bricks, the weakest gave a result of 564.8 tons per square foot, and the strongest 788 tons; the mean of the four being 701.1 tons per square foot. The results on the Leicester “reds” are no more encouraging; the figures in the foregoing tables are 150.6 tons, 229 tons, 308 tons, and 337 tons per square foot. Similarly, Professor Unwin has more recently found that the Leicester “reds” from Elliston, near Leicester, bear a crushing strain varying from 311.4 tons to 591.4 tons per square foot in four samples.