[73] See above, p. 138.
[74] These quantities are the result of the experiments made in September 1847. They agree with what is now the received opinion of authorities on train resistances, and represent favourably the case for the locomotives at the time of Mr. Brunel’s report in August 1844. At the time when Mr. Brunel wrote his report of August 1844, the weight of a locomotive, as has been said, bore a higher ratio to its power.
[75] It must be borne in mind that all the inconveniences attending the use of auxiliary locomotives must be encountered, or else the excessive dead weight of an engine powerful enough to take a train up the steepest gradient in a hilly district must accompany it for the whole length of that part of the line.
[76] No dynamometer was used in these experiments, but all other requisite data were recorded with the greatest exactness, and the horse-power employed may be deduced by means of the scale of resistance which the subsequent dynamomotric trials supply. Moreover, the result above arrived at for the consumption of coke is verified by an examination of published indicator diagrams taken off the same engine on another occasion.
[77] It would of course be impossible here to give a description of all Mr. Brunel’s bridges, or even to refer to the most important of them with that minuteness which would be required if this were a book written for professional use. The following publications may be consulted:—Bourne’s History and Description of the Great Western Railway, 1846; Brees’ Railway Practice, 1837; Simms’ Public Works of Great Britain, 1838; Proceedings of Institution of Civil Engineers, vols. 14, 25, and generally; Molinos et Pronnier, Construction des Ponts Métalliques, 1857; Humber’s Cast and Wrought-Iron Bridge Construction, 1861; and Humber’s Record of Engineering for 1866. At the end of the description of many of the bridges in this chapter a note has been given of publications in which the bridge has been referred to.
[78] In the early days of the Great Western Railway special designs were made for every one of the ordinary bridges over and under the railway; but when, in consequence of the rapid extension of the Great Western system, the number of bridges to be designed became very large, Mr. Brunel had a set of ‘standard drawings’ prepared and engraved, which embodied the experience gained, and contained designs suitable for various situations. The contract drawings were made by adapting to the particular circumstances of each case the standard drawing which was most applicable to it. This system, besides securing uniformity of construction, introduced a considerable amount of economy; since, the standard drawings being based upon the results arrived at in an extensive practice, the proper structural arrangements and dimensions were indicated with far greater accuracy than could be attained in a reasonable time by an independent calculation in each individual case.
[79] It was called the ‘Wharncliffe Viaduct,’ in acknowledgment of the services rendered to the Company by the late Lord Wharncliffe as Chairman of the Committee in the House of Lords. Drawings of this bridge are given in Simms’ Public Works of Great Britain, 1838, pl. 54, 55, and 56; and in Bourne’s History and Description of the Great Western Railway, 1846.
[80] At the back of retaining walls, such as the abutments and wing walls of bridges which were subject to the pressure of earth behind them, Mr. Brunel introduced what were termed ‘sailing courses,’ projecting shelves corbelled out at the back of the wall. The weight of earth resting on these shelves virtually increased the weight of the back of the wall, and assisted it in resisting the forward pressure of the earth.
[81] During the construction of the bridge a part of the crown of the eastern arch proved defective, in consequence of the cement in the middle of the brickwork not having set sufficiently at the time when the centering was eased. Apprehensions which had been entertained by some as to the safety of the structure were groundless, for when the defective part was taken out and replaced, no further trouble was experienced. The bridge has stood well, and has shown none of those symptoms which an overstrained structure exhibits.
[82] Simms’ Public Works of Great Britain, pl. 57, 58; Bourne’s History and Description of the Great Western Railway, p. 36.