All About Engines

i6 Ail About Engines at 30 lb. on the square inch is still capable of doing work. Besides, it would never do to play with steam at 150 lb. on the square inch in the first chapter ! And, for reasons which cannot be given here, the slide valve will not allow of steam being cut off so early as one-fifth of the stroke.' The outside lap, therefore, delays the admission of steam to the cylinder, and cuts it off earlier. In other words, the outside lap controls the admission of the steam, and the larger the overlapping feet the longer time are the ports closed during each stroke of the piston. The reader will probably be wondering by now why the author has been so stupid as to bore him with this minute analysis of valve motion so early in the book ; and if he can keep a secret he shall be told. There are a good many boys, and even young men, who set out to make an engine and whQ succeed admirably up to a certain point. The workman- ship is good, the joints are tight, and every part is polished in a way that reflects the patience and loving care which has been bestowed upon the work. But when steam is admitted the engine refuses to work, and—whisper this low—they have to get an engineer to set the valve for them ! Now this is perfectly unnecessary. A boy who has the skill and perseverance to make an engine has the intelligence to adjust it, and if he really wishes to know how it works he should construct the model shown in Fig. 11 and spend a few hours in making experiments with it. The black parts are