High-pressure Engines. -- Leupold's Engine. -- Trevithick and Vivian. -- Effects of Improvement in Locomotion. -- Historical Account of the Locomotive Engine. -- Blenkinsop's Patent. -- Chapman's Improvement. -- Walking Engine. -- Stephenson's First Engines. -- His Improvements. -- Liverpool and Manchester Railway Company. -- Their Preliminary Proceedings. -- The Great Competition of 1829. -- The Rocket. -- The Sanspareil. -- The Novelty. -- Qualities of the Rocket. -- Successive Improvements. -- Experiments. -- Defects of the Present Engines. -- Inclined Planes. -- Methods of surmounting them. -- Circumstances of the Manchester Railway Company. -- Probable Improvements in Locomotives. -- Their capabilities with respect to speed. -- Probable Effects of the Projected Railroads. -- Steam Power compared with Horse Power. -- Railroads compared with Canals. (80.) 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 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_. We are not, however, to understand that every engine, in which steam is used of a pressure exceeding that of the atmosphere, is what is meant by a _high-pressure engine_; for in the ordinary engines in common use, constructed on Watt's principle, the safety-valve is loaded with from 3 to 5 lbs. on the square inch; and in Woolf's engines, the steam is produced under a pressure of 40 lbs. on the square inch. These would therefore be more properly called _condensing engines_ than _high-pressure engines_; a term quite inapplicable to those of Woolf. In fact, by _high-pressure engines_ is meant engines in which no vacuum is produced, and, therefore, in which the piston works against a pressure equal to that of the atmosphere.