Views of an ambitious kind gradually opened to him, and, by zealously prosecuting his professional labours in summer, he was enabled to spend the winter in Edinburgh, where he attended the lectures of professor Robison on natural philosophy, and those of Dr Black on chemistry. Having thus fitted himself in some measure for the profession of an engineer, he proceeded to Soho, with a recommendation from Robison to Messrs Bolton and Watt. On the way, he examined the aqueduct bridge at Lancaster, the docks at Liverpool, and the interesting works on the Bridgewater canal. At Soho, he was immediately taken into employment, and it was not long ere Mr Watt discovered the extraordinary talents of his young assistant. In the erection of the Albion mills in London, which was completed in 1789, Mr Rennie was intrusted by his employers with the construction of the mill-work and machinery, which were admitted to be of superior excellence. These mills consisted of two engines, each of fifty horse power, and twenty pairs of millstones, of which twelve or more pairs, with the requisite machinery, were constantly kept at work. In place of wooden wheels, so subject to frequent derangement, wheels of cast-iron, with the teeth truly formed and finished, and properly proportioned to the work, were here employed; the other machinery, which used to be made of wood, was made of cast-iron in improved forms. This splendid establishment, which Mr Watt acknowledges to have formed the commencement of the modern improved system of mill-work, was destroyed in 1791, by wilful fire, being obnoxious to popular prejudices, under the mistaken supposition of its being a monopoly. The mechanism, however, established Mr Rennie’s fame, and he soon after began to obtain extensive employment on his own account.

The earlier years of his professional life were chiefly spent in mill-work; and his merits in this line may be briefly stated. One striking improvement was in the bridge-tree. It was formerly customary to place the vertical axis of the running mill-stone in the middle of the bridge-tree, which was supported only at its two extremities. The effect of this was that the bridge-tree yielded to the variations of pressure arising from the greater or less quantity of grain admitted between the mill-stones, which was conceived to be an useful effect. Mr Rennie, however, made the bridge-tree perfectly immovable, and thus freed the machinery from that irregular play which sooner or later proves fatal to every kind of mechanism. Another improvement by Mr Rennie has been adverted to in the above account of the Albion mills; but the principal one was in the comparative advantage which he took of the water power. He so economized the power of water as to give an increase of energy, by its specific gravity, to the natural fall of streams, and to make his mills equal to fourfold the produce of those, which, before his time, depended solely on the impetus of the current.

Mr Rennie was gradually attracted from the profession of a mechanician to that of an engineer. In the course of a few years after his first coming into public notice, he was employed in a considerable number of bridges and other public works, all of which he executed in a manner which proved his extraordinary genius. His principal bridges are those of Kelso, Leeds, Musselburgh, Newton-Stewart, Boston, and New Galloway. The first, which was erected between 1799 and 1803, has been greatly admired for its elegance, and its happy adaptation to the beautiful scenery in its neighbourhood. It consists of a level road-way, resting on five elliptical arches, each of which has a span of seventy-three feet, and a rise of twenty-one. The bridge of Musselburgh is on a smaller scale, but equally perfect in its construction. A remarkable testimony to its merits was paid in Mr Rennie’s presence, by an untutored son of nature. He was taking the work off the contractor’s hands, when a magistrate of the town, who was present, asked a countryman who was passing at the time with his cart, how he liked the new bridge. "Brig," answered the man, "it’s nae brig ava; ye neither ken whan ye’re on’t, nor whan ye’re aff’t" It must be remarked that this bridge superseded an old one in its immediate neighbourhood, which had a very precipitous road-way, and was in every respect the opposite of the new one.

Mr Rennie was destined, however, to leave more splendid monuments of his talents in this particular department of his profession. The Waterloo bridge across the Thames at London, of which he was the architect, would have been sufficient in itself to stamp him as an engineer of the first order. This magnificent public work was commenced in 1811, and finished in 1817, at the expense of rather more than a million of money. It may safely be described as one of the noblest structures of the kind in the world, whether we regard the simple and chaste grandeur of its architecture, the impression of indestructibility which it forces on the mind of the beholder, or its adaptation to the useful purpose for which it was intended. It consists of nine equal arches, of 127 feet span; the breadth between the parapets is 42 feet; and the road-way is perfectly flat. Mr Rennie also planned the Southwark bridge, which is of cast. iron, and has proved very stable, notwithstanding many prophecies to the contrary. The plan of the new London bridge was likewise furnished by him; but of this public work he did not live to see even the commencement.

Among the public works of different kinds executed by Mr Rennie may be mentioned;—of canals, the Aberdeen, the Great Western, the Kennet and Avon, the Portsmouth, the Birmingham, and the Worcester;—of docks, those at Hull, Leith, Greenock, Liverpool, and Dublin, besides the West India docks in the city of London;—and of harbours, those at Berwick, Dunleary, Howth, Newhaven, and Queensferry. In addition to these naval works, he planned various important improvements on the national dockyards at Plymouth, Portsmouth, Chatham, and Sheerness; and the new naval arsenal at Pembroke was constructed from his designs. But by far the greatest of all his naval works was the celebrated breakwater at Plymouth. It is calculated that he planned works to the amount of fifty millions in all, of which nearly twenty millions were expended under his own superintendence.

Mr Rennie died, October 16, 1821, of inflammation in the liver, which had afflicted him for some years. By his wife, whom he married in 1789, he left six children, of whom the eldest, Mr George Rennie, followed the same profession as his father. This eminent man was buried with great funeral honours, in St Paul’s cathedral, near the grave of Sir Christopher Wren.

The grand merit of Mr Rennie as an engineer is allowed to have been his almost intuitive perception of what was necessary for certain assigned purposes. With little theoretical knowledge, he had so closely studied the actual forms of the works of his predecessors, that he could at length trust in a great measure to a kind of tact which he possessed in his own mind, and which could hardly have been communicated. He had the art of applying to every situation where he was called to act professionally, the precise form of remedy that was wanting to the existing evil,—whether it was to stop the violence of the most boisterous sea—to make new harbours, or to render those safe which were before dangerous or inaccessible—to redeem districts of fruitful land from encroachment by the ocean, or to deliver them from the pestilence of stagnant marsh--to level hills or to tie them together by aqueducts or arches, or, by embankment, to raise the valley between them—to make bridges that for beauty, surpass all others, and for strength seem destined to last to the latest posterity—Rennie had no rival. Though he carried the desire of durability almost to a fault, and thus occasioned more expense, perhaps, on some occasions, than other engineers would have considered strictly necessary, he was equally admired for his conscientiousness in the fulfilment of his labours, as for his genius in their contrivance. He would suffer no subterfuge for real strength to be resorted to by the contractors who undertook to execute his plans. Elevated by his genius above mean and immediate considerations, he felt in all his proceedings, as if he were in the court of posterity: he sought not only to satisfy his employers, but all future generations.

Although Rennie did not devote himself to the acquisition of theoretical knowledge, excepting to that general extent which is required by every well-informed engineer, he was fond of those investigations of a mixed character, where the results of experiment are combined by mathematical rules, and a train of inquiry directed and modified by the lights of theory. In his instrument for ascertaining the strength of flowing water, he has made a contribution to science of no small importance.