His first Experiments to realise these Inventions 125 His experimental Apparatus 125 His experimental Models fitted up at Delft House, in Glasgow 128 Difficulties of bringing the improved Engines into Use 129 Watt first employed by Roebuck as a Civil Engineer 130 His Partnership with Roebuck 130 His first Patent 130 Experimental Engine at Kinneal 131 Abstract of the Act of Parliament for the Extension of his Patent 132 Description of his single-acting Steam Engine 133 CHAP. VI. WATT'S ENGINES. Correspondence of Watt with Smeaton 145 Failure of Condensation by Surface 146 Improvements in Construction of Piston 147 Method of Packing 148 Improvements in boring the Cylinder 149 Disadvantages of the new compared with the old Engines 150 Greatly increased Economy of Fuel 150 Economy of the Engine 151 Expedients to force the new Engines into Use 151 Correspondence of Boulton 153 Correspondence with Smeaton 155 Efficiency of Fuel in the new Engines 156 Discovery of the expansive Action of Steam 157 Watt states it in a Letter to Dr. Small 157 Its Principle explained 158 Mechanical Effects resulting from it 162 Computed Effect of cutting off Steam at different Portions of the Stroke 163 Produces a variable Power 163 Expedients for equalising the Power 164 Expansive Principle in Watt's Engines limited 165 Its more extensive Application in the Cornish Engines 165 CHAP. VII. DOUBLE-ACTING ENGINE. Common Steam 168 Superheated Steam 170 Laws of Dalton and Gay Lussac 171 Law of Mariotte 171 Relation between Temperature and Pressure of common Steam 171 Effects of the Expansion of common Steam 173 Mechanical Effects of Steam 173 Method of equalising the expansive Force 174 Hornblower's Engine 175 Woolf's Engine 176 Watt's Attempts to extend the Steam Engine to Manufactures 178