RENNIE, JOHN, is a member of a family well worthy of admiration in all that respects mechanical and engineering skill. He was horn in 1761 in Haddingtonshire. He ap pears to have very early devoted his attention to the subjects of machinery and architecture, and he attended at Edinburgh the lectures given by Dr. Robinson and Dr. Black, on mechanical philosophy and chemistry. About 1780 he left his native place, and shortly afterwards established himself as a mechanist in London, where he obtained immediate employment. From this period be continued to be occupied in the construction of steam engines, or of the different kinds of machinery to which, as a first mover, steam is applied ; and at the same time he was almost con stantly engaged in designing or superintend ing those public works which have given him so just a to celebrity. Between 1799 and 1803 he constructed the stone bridge at Kelso, below the junction of the Tweed and Teviot. He also built stone bridges at Mus selburgh and other places in Scotland ; but his masterpiece of this kind is the Waterloo Bridge over the Thames. The. Southwark Bridge, over the same river, is an iron bridge, and was also constructed by him. Lfe super intended the formation of several canals. But his chief work in connection with inland navigation is the Kennet and Avon Canal, which extends from Bath to Newbury. He also gave a plan for draining the fens at Witham in Lincolnshire, which was executed in 1812. The London Docks and the East and West India Docks at Blackwall are among the great works which are executed from the plans and under the direction of Mr. Rennie. Ile formed the new docks at Hull (where also he constructed the first dredging machine used in this country), the Prince's Dock at Liverpool, and the docks of Dublin, Greenock, and Leith. To these must be added. the Plymouth Breakwater. Mr. Rennie also gave plans for improving the harbours of Berwick, Newhaven, and other places, and the dockyards of Portsmouth, Plymouth, Pembroke, and Chatham : he also built the pier at Holyhead. Before his death he had given plans for improving the docks at Sheerness ; which have since been exe cuted by his first and second eons, Messrs.
George and John (now Sir John) Rennie. To Sir John Bennie was also confided. the charge of the construction of the present London Bridge, from the design of his father.
Mr. Rennie died in 1821, and was buried in St. Paul's Cathedral, His sons are worthily following in his steps in engineering works.
ItBPULSION. Both attraction and repul sion exist in all the particles of material sub stances, and seem to be properties by which those particles act upon one another when not in contact. The reality of a substance between the particles of bodies, whether solid, fluid, or gaseous, admits of no question; for the difference in the densities of these classes of bodies, as wall as the different contractions of the bodies in the process of cooling, can only be conceived to arise from the different extent of the intervals between the particles. It is right however to observe, that the word repplsion is often applied to phenomena which are in reality the results of attraction. For example, a small quantity of quicksilver being laid on a glass plate assumes a spherical form, instead of spreading over it in a thin sur face; and this was once supposed to arise from a repulsive power in the glass, whereas it is owing to the attraction of the particles of quicksilver for one another being greater than the attraction of the glass for the quick silver, The expansions of •solids and fluids by heat, and the elastic powers of gas at dif ferent temperatures, are consequences of the repulsions residing in the particles of caloric, or Induced by the latter in those of the bodies with which they are combined. The repul sive power existing in the air which is con densed in nitre, produces, on being combined with heat, a velocity of expansion equal to about 7000 feet per second ; and the force of pressure resulting from it is thought to be equal to 2000 times the pressure of the atmos phere. It is this which gives such force to the explosion of gunpowder.
The scientific theories whereby the pheno mena of expulsion are sought to be explained, and the experiments made to test the theories, are beyond our present scope.