Page:A biographical dictionary of eminent Scotsmen, vol 7.djvu/33

Rh he undertook on his own account was the rebuilding of the flour-mills at Invergowrie, near Dundee.

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 mi 11- 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 tcsti-