SPECIAL REPORT OF MR. HILL, ON MACHINERY. 415
in this way. It is to be borne in mind, that the leading English firms have for a long time been in the habit of consulting the scientific men of their technical schools, and have adopted the principle of steam-jacketing after a most thorough understanding of its advantages, which are a subject of the most exact mathematical demonstration, depending upon the degree of expansion. Following their lead, the principle has become one of general adoption among the English engineers. It has equally come into use with the leading continental manufacturers, who study economy of fuel, and we believe all the variable engines of the Corliss type and many with slide- valves, were thus built. The marine and boat engines, it is needless to say, were all designed in this way.*
The marine engines were, in addition to this, With one exception, on the compound principle. The attention that this subject is now receiving from our steam engineers, the fact that it is in use on all the principal transatlantic lines, and that the United States navy and some of our leading lines have adopted it, led the writer to lpok into the subject as carefully as circumstances would permit.
The steamer by which passage was taken across the Atlantic, was one of a line whose managers are commonly considered among the most conservative and slow to adopt new ideas of any in Europe, but they have accepted compound engines, and are, as fast as possible, placing them in all their steamers. It was said to be the declaration of the chief engineer of their line, that after going through all changes and improvements which had been made for twenty years past, their books showed but little variation in the consumption of coal until the introduction of compound engines, when the saving ap-
* Although the principle is well understood by engineers, the writer has met so many persons who suppose that a steam-jacket was merely a method of protecting a cylinder from outside radiation, (better done by felt and cleating or lagging), that he ventures to explain, that it is not for this purpose at all, but to correct the loss from condensation of steam within thé cylinder, which condensation is a consequence of the cooling effect of a rapid rate of expansion when the cut-off valve is closed. The expansion of a volatile liquid by means of an air-pump will freeze water; in the same way the expanding steam robs part of itself of its heat, and condenses it into water, thus diminishing the pressure more than it should, and at the end of the stroke robbing the cylinder of heat by again turning the water condensed on its surface into steam, which flows directly into the exhaust without doing any work, leaving the surface of the cylinder cool to be warmed by the fresh steam at the cost of fresh condensation. The steam in the jacket supplies the heat to prevent this, at a less loss than if thus washed in the cylinder.