+======================================+==========+=========+========+ | | | | Per- | | | | | centage| | | Initial | | of | | | Pressure | Total | saving | | | of steam | heat of |compared| | | in pounds| steam | with | | Period of admission or point of |per square| used, | full | | cut-off. | inch. |in units.| stroke.| +--------------------------------------+----------+---------+--------+ |Full stroke | 70. | 527. | | |18 in. = Three-quarters of the stroke,| 72.5 | 408.7 | 22¹⁄₂ | |12 in. = One-half „ „ | 82.7 | 309.5 | 41¹⁄₄ | | 8 in. = One-third „ „ | 100. | 247.5 | 53 | | 6 in. = One-quarter „ „ | 117.4 | 215.9 | 58 | | 4 in. = One-sixth „ „ | 150.5 | 186.5 | 64¹⁄₂ | | 3 in. = One-eighth „ „ | 181.8 | 165.8 | 68¹⁄₂ | | 2 in. = One-twelfth „ „ | 241.4 | 144.8 | 72¹⁄₂ | +--------------------------------------+----------+---------+--------+ From this table it will be seen that theoretically 22¹⁄₂ per cent. of heat is saved by cutting off at ³⁄₄ of the stroke and using steam of 72.5 lbs. pressure instead of steam of 70 lbs. worked full stroke. Cutting off at half stroke and using steam of 82.7 lbs., 41¹⁄₄ per cent. of heat is saved, and cutting off at quarter stroke with steam of 117.4 lbs. saves 58 per cent. of heat; and at one-twelfth of the stroke, or expanding steam of 241.4 lbs. pressure to twelve times its volume, saves 72¹⁄₂ per cent. of heat. As stated before, the above is the _theoretical_ advantage of using steam expansively. There are, however, practical difficulties in the way of using some of these high degrees of expansion. It has already been explained that if steam is cut off early in the stroke and the degree of expansion increased, the pressure and consequently the temperature of the steam must also be increased. The danger of explosion is greater with the higher pressures, and stronger and more expensive boilers and machinery are therefore needed. With steam of very high temperature the metal of the cylinders, pistons and valves becomes so much heated that they soften, and then the friction of the one on the other causes them to cut or scratch each other. The high temperature at the same time destroys the oil or other lubricant used in contact with the steam. It is also impossible to admit and cut off steam very early in the stroke with the ordinary mechanical appliances used for moving slide-valves of locomotives. This latter difficulty will be more fully explained hereafter. PART VI.