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 an _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 _low-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. In these engines the whole of the condensing apparatus, viz. the cold-water cistern, condenser, air-pump, cold-water pump, &c., are dispensed with, and nothing is retained except the boiler, cylinder, piston, and valves. Consequently, such an engine is small, light, and cheap. It is portable also, and may be moved, if necessary, along with its load, and is therefore well adapted to locomotive purposes. (181.) High-pressure engines were one of the earliest forms of the steam engine. The contrivance, which is obscurely described in the article already quoted (7.), from the Century of Inventions, is a high-pressure engine; for the power there alluded to is the elastic force of steam working against the atmospheric pressure. Newcomen, in 1705, applied the working-beam, cylinder, and piston to the atmospheric engine; and Leupold, about 1720, combined the working-beam and cylinder with the high-pressure principle, and produced the earliest high-pressure engine worked by a cylinder and piston. The following is a description of Leupold's engine:— [Pg323] [Illustration: _Fig._ 82.]