CATALOGUE OF THE MECHANICAL ENGINEERING COLLECTION. 15 



waste in the engine arose from cooling of the cylinder and piston 

 surfaces by the water spray used to condense the steam. To reduce 

 this waste, Watt invented the separate condenser, which when in- 

 stalled on a Newcomen engine cut the fuel consumption in half. 

 He likeAvise covered the top of the cylinder to exclude the cold air 

 and exposed it to the steam. These improvements, together with the 

 air pump, were embodied in the patent secured by Watt in 1769 

 and extended in 1775 for a period of 25 years, just at the time when 

 he entered into partnership Avith Matthew Boulton for the manu- 

 facture of single-acting beam pumping engines. 



The changes Watt made in the construction of the engine rendered 

 it almost a new machine. The most important of these improve- 

 ments were: 



(1) Surrounding the steam cylinder with a jacket, inclosing the 

 whole in a casing, and doing away with the necessity of introduc- 

 ing into the cylinder v.'ater or other substances colder than the steam 

 itself. 



(2) Condensing the steam in a separate vessel. 



(3) Removal of the uncondensed air or vapor by an air pump. 



(4) Substitution of the expansive force of steam instead of the 

 pressure of the atmosphere acting on the piston, whereby the engine 

 ceased to be an atmospheric and became a steam engine. 



(5) Use of grease to render the piston steam-tight instead of water, 

 as formerly employed. 



(6) Adaptation of the principle of expansion by cutting off the 

 steam before the piston had finished its stroke. 



(7) Introduction of the double-acting engine. 



(8) The mechanism known by the name of "parallel motion." 

 This was first designed for the double-acting engine to replace the 

 toothed rack and sector required for giving an upward as well as a 

 downward propulsion. It was afterwards applied to single-acting 

 engines, being a much more suitable method of connecting the piston 

 rod to the beam than the original arc and chain. 



All of these inventions were protected by patents, but, independ- 

 ently of those thus secured. Watt made other alterations of very 

 considerable value. Thus he changed the dome boiler formerly used 

 with the atmospheric engine to the wagon-shaped one, which has 

 since been applied generally to low-pressure engines. By this change 

 more heating surface was gained for a given cubic content, the boiler 

 was more easily made, and was of a more convenient form for setting. 

 He also placed the boiler upon a separate foundation, removing it 

 from beneath the cylinder. This was a great improvement, for in 

 the atmospheric engine, the cylinder of which stood on the top of 

 the dome boiler, the vibration caused by the motion of the engine soon 

 rendered the boiler leakj^ and deranged the position of the cylinder. 



