[28] Those who desire to investigate this controversy more in detail will find very full information on the subject in the Translation of Arago's Eloge, with notes and appendix by J. P. Muirhead, Esq. Murray, London, 1839. [29] An account of this remarkable apparatus, accompanied by an engraving made from a drawing supplied by Watt, was communicated by Sir John Robison to the _Edinburgh Philosophical Journal_ in 1820. _See_ vol. iii, p. 60. [Pg321] [Illustration] CHAP. XI. LOCOMOTIVE ENGINES ON RAILWAYS. NON-CONDENSING ENGINES. — LEUPOLD'S ENGINE.-TREVETHICK AND VIVIAN. — EFFECTS OF RAILWAY TRANSPORT. — HISTORY OF THE LOCOMOTIVE ENGINE. — BLENKINSOP. — MESSRS. CHAPMAN. — WALKING ENGINE. — MR. STEPHENSON'S ENGINES AT KILLINGWORTH. — LIVERPOOL AND MANCHESTER RAILWAY. — EXPERIMENTAL TRIAL. — THE ROCKET. — THE SANSPAREIL. — THE NOVELTY. — SUBSEQUENT IMPROVEMENTS IN THE LOCOMOTIVE ENGINE. — LARDNER'S EXPERIMENTS IN 1832. — ADOPTION OF BRASS TUBES. — MR. BOOTH'S REPORT. — DETAILED DESCRIPTION OF THE MOST IMPROVED LOCOMOTIVE ENGINES. — POWER OF LOCOMOTIVE ENGINES. — EVAPORATION OF BOILERS. — LARDNER'S EXPERIMENTS IN 1838. — RESISTANCE TO RAILWAY TRAINS. — RESTRICTIONS ON GRADIENTS. — COMPENSATING EFFECT OF GRADIENTS. — EXPERIMENT WITH THE HECLA. — METHODS OF SURMOUNTING STEEP INCLINATIONS. (180.) In the various modifications of the steam engine which we have hitherto considered, the pressure introduced on one side of the piston derives its efficacy either wholly or partially from the vacuum produced by condensation on the other side. This always requires a condensing apparatus, and a constant and abundant supply of cold water. An engine of this kind must therefore necessarily have considerable dimensions and weight, and is inapplicable to uses in which a small and light machine only is admissible. If the condensing apparatus be dispensed with, the piston will always be resisted by a force equal to the atmospheric [Pg322] pressure, and the only part of the steam pressure which will be available as a moving power, is that part by which it exceeds the pressure of the atmosphere. Hence, in engines which do not work by condensation, steam of a much higher pressure than that of the atmosphere is indispensably necessary, and such engines are therefore called _high-pressure engines_.