25. Sectional and Front Elevations of One of the Two Pairs of Porter-Allen Engines in the Colt Armory, Hartford, Conn. 180 26. Sectional and Side Elevations of One of the Two Pairs of Porter-Allen Engines in the Colt Armory, Hartford, Conn. 181 27. Porter-Allen Engines in the Colt Armory, Hartford, Conn. Front View 181 28. Porter-Allen Engines in the Colt Armory, Hartford, Conn. Rear View 181 29. Surface Plates Designed by Mr. Porter 182 30. Diagram from Allen Engine, Back End of Cylinder, at Fair of American Institute, 1870 194 31. Friction Diagram from Allen Engine at Fair of American Institute, 1870 196 32. Diagram from Allen Engine, Fair of American Institute, 1870, Cutting Off at ¹⁄₄ Stroke 196 33. Apparatus for Graphically Showing the Acceleration and Retardation of the Reciprocating Parts of an Engine 205 34. The Allen Boiler _Facing_ 208 35. The Prototype of the Modern High-speed Engine, Fly-wheel Side 223 36. Prototype of the Modern High-speed Engine, Crank Side 224 37. Longitudinal Section of Cylinder and Valves 225 38. Cross-section of Cylinder and Valves 226 39. Connections of Admission Valves 226 40. First Arrangement of Exhaust Valves 228 41. Main Bearing 230 42. Eccentric and Cross-head and Crank-pin Lubricators 230 43. Surface Plate for Producing a True Plane 234 44. Mr. Porter’s Regulating Valve 244 45. The Corliss Engine Exhibited at the Centennial Exhibition 249 46. Porter-Allen Engine Equal in Power to the Exhibited Corliss Engine 250 47. Mr. Porter’s Fly-wheel 269 48. Connection of Arms and Rim in Mr. Fritz’ Fly-wheel 273 49. Mr. Allen’s Patent Pressure Plate 293 50. Diagrams from the Otis Engine 311 51. Otis Engine. Dash Pot for Governor 313 52. Diagrams from my First and Only Compound Engine 318 LIST OF FULL-PAGE HALF-TONE PORTRAITS PAGE