468 POPULAR LECTURES AND ADDRESSES. 



droppings down of temperature, between the 

 highest temperature to which the water of the 

 boiler can be raised for safe and effective use, and 

 the temperature of the exhaust steam. The 

 superior efficiency of a condensing engine consists 

 in its allowing the temperature of the exhaust 

 steam to be about 40 or 50 C., instead of its 

 being a degree or two above 100, as it essentially 

 is in the non-condensing expansive engine. James 

 Watt was, by his separate condenser, his use of 

 expansion in single cylinder engines, and his 

 origination of the now generally employed plan 

 of double, or triple, or quadruple expansion engine, 

 with his perfect tact and judgment as to practical 

 economy, and his profound scientific knowledge 

 of mechanics and of the properties of steam, 

 arranging his engine to as nearly as possible fulfil 

 Carnot's condition of reversibility, by minimising 

 every irreversible action in its cycle of work. But 

 it seems certain that he had no idea of Carnot's 

 grand generalisation, according to which one 

 perfectly reversible engine would give exactly as 

 much work as any other, of whatever different 



