1870.] Notices of Scientific Works. 73 



Wrought-iron girders, whether tubular, plate, or lattice, are 

 therefore simply modifications of what has already been done in the 

 same direction, and provided careful attention is given to the quality 

 of the material and soundness of the workmanship, we may rest 

 assured of the security and permanence of every similar structure. 



Many distinguished men of science have written treatises to 

 elucidate the principles involved in the tubular and girder con- 

 structions, and among them may be mentioned Mr. Unwin, who 

 has just published a report of his lectures on this subject, including 

 a short treatise on the construction of iron roofs, delivered to the 

 members of the Ordnance Corps at Chatham. As a lecturer on 

 practical science a more efficient person could probably not have 

 been selected, for — to a competent knowledge of mathematics ap- 

 plied to constructions — Mr. Unwin has had the advantage of five 

 years' practical experience with Sir William Fairbairn of Man- 

 chester, and he therefore enters upon the discussion with a full 

 knowledge of the subject on which he treats. 



In the first lecture Mr. Unwin fully proves the advantages 

 he has gained, by the competency with which he enters upon the 

 investigation of stress and strain, bringing to his aid many apt 

 illustrations, and by the graphic way in which he treats the 

 principles of load and molecular resistances, greatly for the benefit 

 of the student and those interested in the accuracy of iron 

 constructions. To show the distinction between stress and strain, 

 which means the force of the former applied to a material body, 

 and the alteration or the resistance of the latter as a result, 

 Mr. Unwin states, that the "strain is sensibly proportional to 

 the stress for a range of about only one-third of the whole stress 

 which may be applied before rupture ensues, if the bar has not 

 previously been strained, and for a range of, perhaps, two-thirds of 

 the breaking-stress, if the bar has not previously been loaded with 

 nearly the whole breaking-weight. But in either case, with loads 

 near the breaking-weight, the strain is not proportional to the 

 stress, and the condition of perfect elasticity is not fulfilled. Hence 

 laws derived from the consideration of a perfectly elastic material 

 will not give accurately the ultimate resistance of structures of 

 wrought iron." 



The second lecture is devoted to the intensity of stress on 

 bridges, and the methods of estimating the load and its limits of 

 safety. It also relates to the weight of the structure and its load, 

 or the dead weight and its live or rolling load. In this, as in the 

 former lecture, Mr. Unwin gives some useful examples and illustra- 

 tions of highly practical value to the student and engineer. 



The third lecture treats of tubular, tubular-girder, and plate- 

 girder bridges, showing the ratio of the top and bottom flanges to 

 span, the depth of girder to span, and the method of designing, &c. 



