Young Rennie worked eight hours a day in the practical part of his profession, and spent four in the acquisition of science and the modern languages, aided in both by the first men in London in their branches. Four or five years of this training gave him, as he says in his autobiography, the "_rudiments_" of his profession. His father next determined to give him some experience in bearing responsibility, and placed him as an assistant to the resident-engineer of Waterloo Bridge, then in course of construction. He was but nineteen years of age; but, being the son of the head of the firm, he was naturally deferred to and prepared to take the lead. Soon after, the Southwark Bridge was begun, which the young man superintended daily at every stage of its construction.
English engineers regard this bridge as the _ne plus ultra_ of bridge-building. A recent writer speaks of it as "confessedly unrivaled as regards its colossal proportions, its architectural effect, or the general simplicity and massive character of its details." It crosses the river by three arches, of which the central one has a span of two hundred and forty feet, and it is built at a place where the river at high tide is thirty-six feet deep. The cost of this bridge was four millions of dollars, and it required five years to build it. The bridge is of iron, and contains a great many devices originated by the young engineer, and sanctioned by his father. It was he also who first, in recent times, learned how to transport masses of stone of twenty-five tons weight, used for the foundation of bridges.
Having thus become an accomplished engineer, his wise old father sent him on a long tour, which lasted more than two years, in the course of which he inspected all the great works, both of the ancients and moderns, in Europe, and the more accessible parts of Africa and Asia. Returning home, the death of his father suddenly placed upon his shoulders the most extensive and difficult engineering business in Great Britain. But with such a training, under such a father, and inheriting so many traditional methods, he proved equal to the position, continued the great works begun by his father, and carried them on to successful completion.