104 Britain's Heritage of Science 



gating for. himself the connexion between its temperature, 

 density, and pressure. Instead of allowing the steam to 

 pass into the cylinder during the whole of the stroke, Watt 

 saw that a considerable economy could be effected by 

 stopping the admission when the stroke had reached a 

 certain point and allowing the pressure of the steam already 

 in the cylinder to complete it. It is not necessary to enter 

 further into the many improvements of detail which the 

 steam engine owes to Watt, who, realizing the future that 

 was before it, also devised various means by which the up 

 and down stroke of the engine could be converted into 

 rotatory motion. 



Savery is said to have been the first to suggest that the 

 measured power of performance of an engine might be in 

 terms of horse-power, but Watt actually investigated the work 

 that a horse could do in a given time, and defined one horse- 

 power as the rate at which work is done when 33,000 Ibs. 

 are raised one foot in one minute. 



Watt was of a retiring disposition, due, no doubt, to 

 the weakness of health which, in the early part of his life, 

 greatly interfered with his work. He speaks of himself as 

 " indolent " and " not enterprising," and as being " out 

 of my sphere when I have anything to do with mankind." 

 His inventions were not confined to the steam engine. He 

 constructed a press for copying manuscripts, such as is now 

 in common use. It is also claimed on behalf of Watt, 

 and with some justification, that he was the first to discover 

 the true composition of water as a compound of oxygen 

 and hydrogen. The controversy which arose has already 

 been referred to (page 85). 



The condenser used by Watt can be easily attached to 

 stationary engines, but is inconvenient when an economy 

 of space is imperative, as when steam is used for road 

 propulsion. The condenser may then be dispensed with, 

 but the pressure of steam has to be increased. Richard 

 Trevithick (1771-1833), whose father was the manager of a 

 Cornish mine, invented a road locomotive with high pressure 

 steam, and conveyed passengers with it on Christmas Eve, 

 1801. Some sort of steam vehicle had, however, already 

 been built in France by Nicolas Cugnot as early as 1769, 



