All About Engines

T44 All About Engines spending figures relating to the first cube. Similarly, if the cube had edges of 3 feet, the area would be 54 square feet, and the volume 27 cubic feet, or nine and twenty-seven times respectively greater than the area and volume of the first cube. While a steam engine cylinder is not, of course, cubical in form, the same general relations between the area and volume of cylinders of different size holds good ; so that it will be clear that surface condensation is of less im- portance in large cylinders than in small ones. Again, in high-speed engines the steam is in con- tact with the metal surfaces for so short a time that the heating and cooling effect is much reduced- The jacket is most effective in small slow-running engines, and in large high-speed engines it is of very little importance at all. In any case the condensation is greater the earlier the cut-off, because the fall in temperature is greater ; so that there is a very seri- ous limit to the advantages obtained by expansive working. For example, in small engines there is no advantage in cutting-off earlier than J stroke, while in large engines the cut-off may be £ stroke, but is more frequently | stroke. The engineer, however, is not beaten yet. He has superheated steam at his disposal. Consider a tall, cylindrical vessel with a little water in the bottom and a light, steam-tight piston above it. If the water is heated the temperature rises and a little vapour is formed which presses against the under side of the piston ; for water yields vapour at all temperatures, and even the vapour from ice exerts