230 M. Arago's Biographical Memoir of James Watt. 



Watt, in fact, resolved an important part of a very difficult 

 problem ; he made out the scale of temperament by the me- 

 thod of pulsations (des battements), at that time little under- 

 stood, and the knowledge of vrhich he could not have obtained 

 except in the profound but very obsciu-e work of Dr Robert 

 Smith of Cambridge. 



Histo7'y of the Steam-Engine. 



We are now arrived at the most brilliant period of the life 

 of Watt ; and I fear also at the most difficult part of my task. 

 The immense importance of the inventions of which I am 

 about to treat, cannot for a moment be doubted ; but possibly 

 I may not succeed in making them clearly understood, with- 

 out going into minute numerical comparisons. That these 

 comparisons, if they do become indispensable, may be readily 

 followed, I shall here state, as briefly as possible, the ab- 

 stract physical principles upon which they must be based. 



As the result of simple change of temperature, water may 

 exist in three perfectly distinct states, — in the solid, the li- 

 quid, and the gaseous state. Below 32° Fahr. water becomes 

 ice, at 212° it is rapidly transformed into vapour, and in all 

 the intermediate degrees it is liquid. 



The careful observation of the points of passage from one 

 of these conditions into another, leads to discoveries of the 

 highest importance, which form the key to the economical 

 appreciations of steam-engines. 



Water is not necessarily warmer than is every kind of ice ; 

 water may be maintained at the temperature of 32° without 

 freezing ; ice may continue at 32° without melting ; but it is 

 very difficult to believe that this water and this ice, both of 

 them at one and the same degree of temperature, diff^er only 

 in their physical properties, and that there is not some ele- 

 ment, apart from water properly so called, which distin- 

 guishes the solid water from the liquid water. A very simple 

 experiment will elucidate this mystery. Mix two pounds* 



* In this illustration, the fractions arising from the diiFerences of the 

 thermometric scales 'are omitted, and hence the figures are only approxi- 

 matively correct. — Editor. 



