M. Arago's Memoir of James Watt. 313 



'owing is the passage in the letter to Dr Small, referred to by M. Arago. 

 The letter is dated 28th May 1769. 



" I mentioned to you a method of still doubling the effect of the steam, 

 and that tolerably easy, by using the power of steam rushing into a 

 vacuum, at present lost. This would do little more than double the 

 effect, but it would too much enlarge the vessels to use it all. It is pecu- 

 liarly applicable to wheel-engines, and may supply the want of a con«» 

 denser where force of steam only is used ; for, open one of the steam 

 valves, and admit steam until one-fourth of the distance b etween it and 

 the next valve is filled with steam ; shut the valve, and tlie steam will 

 continue to expand, and to press round the wheel with a diminishing 

 power, ending in one-fourth of its first exertion. The sum of the series 

 you will find greater than one-half, though only one-fourth steam was 

 used. The power will indeed be unequal, but this can be remedied 

 hy a fly, or several other ways." See Edin. Review for January 1809, 

 p. 320. 



For a full account of Mr Watt's Expansion Engine, and of his patent 

 of 1782, see Robison, Steam-Engine, pp. 126 to 131, and still more par- 

 ticularly, Farey, " Treatise on the Steam-Engine," pp. 339 to 352, where 

 extracts from the specification are given. 



(Note on p. 264.) We claim also for Mr Watt the first application 

 of the crank to engines producing rotatory motions. (See Edin. Re- 

 view for Jan. 1809, p. 321, and Robison, Steam-Engine, p. 134.) And 

 to him we are indebted for the beautiful contrivance of the sun and 

 planet wheels, now little used, but of which we refer our readers to an 

 excellent instance in the steam-engine of the brewery of Messrs Whitbread 

 and Co. in London, where it has been in successful operation for up- 

 wards of fifty years. And there are many others. 



" The copying-press," (p. 266.) For a copy of Mr Watt's specifica- 

 tion of this patent, with engravings of the machines, see the '' Repertory 

 of Arts and Manufactures," vol. i. 1794, p. 13. 



" Heating hy Steam." (p. 266.) The most detailed account of Mr 

 Watt's proceedings in the application of this most useful principle, is to 

 be found in the preface to Buchanan on " the Economy of Heat and 

 Management of Fuel." 



" Watt has been forgotten." (p. 267.) This is not quite correct in 

 point of fact. Mr Watt's claims are accurately set forth in an able and 

 impartial article on Water, in the edition of the Encyclopaedia Britan- 

 nica published in the year 1797. There is a very short account of the 

 matter in Murray's Chemistry, ed. 1806, vol. ii. p. 158; and a most im- 

 perfect one in Nicholson's Chemical Dictionary, first edition, 1795, article 

 Water, and Thomson's Chemistry, second edition, 1804, vol. i. p. 677, 

 in all of which, however, the merit of the discovery is more or less at- 

 tributed to Mr Watt. 



