philosopher, who invented the air-pump; and this, combined with the production of a vacuum by the condensation of steam, was subsequently suggested by Papin. The use of a working-beam could not have been unknown. Nevertheless, the judicious combination of these scattered principles must be acknowledged to deserve considerable credit. In fact, the mechanism contrived by Newcomen rendered a machine which was before altogether inefficient, highly efficient: and, as observed by Tredgold, such a result, considered in a practical sense, should be more highly valued than the fortuitous discovery of a physical principle. The method of condensing the steam by the sudden injection of water, and of expelling the air and water from the cylinder by the injection of steam, are two contrivances not before in use, which are quite essential to the [Pg074] effective operation of the engine. These processes, which are still necessary to the operation of the improved steam engine, appear to be wholly due to the inventors of the atmospheric engine. [Illustration: ATMOSPHERIC ENGINE.] FOOTNOTES: [9] This pipe is represented as proceeding from the force-pipe above the cistern C, in the perspective view of Savery's engine at the head of this chapter. [10] Hot water being lighter than cold, floats on the surface. [11] "Captain" is a title given in Cornwall to the superintendent of the works connected with a mine. [12] As the calculation of the power of an engine depends on the number of square inches in the section of the piston, it may be useful to give a rule for computing the number of square inches in a circle. The following rule will always give the dimensions with sufficient accuracy:—_Multiply the number_ _of inches in the diameter by itself; divide the product by 14,_ _and multiply the quotient thus obtained by 11, and the result_ _will be the number of square inches in the circle_. Thus, if there be 12 inches in the diameter, this multiplied by itself gives 144, which divided by 14 gives 10-4/44, which multiplied by 11 gives 115, neglecting fractions. There are, therefore, 115 square inches in a circle whose diameter is 12 inches. [13] The external cylinder is not represented in the diagram. [14] Also called the _snifting_ valve, from the peculiar noise made by the air and steam escaping from it. [15] Also called the _injection valve_. [Pg075] [Illustration: GREENOCK, IN 1824.] CHAP. III.