26 THE HISTORY OF [SECT. i. 



steam is to be condensed in vessels distinct from the steam vessels or cylinders, 

 although occasionally communicating with them : these vessels I call condensers ; 

 and, whilst the engines are working, these condensers ought at least to be kept as 

 cold as the air in the neighbourhood of the engines, by application of water or 

 other cold bodies. Thirdly ; whatever air or other elastic vapour is not condensed 

 by the cold of the condenser, and may impede the working of the engine, is to be 

 drawn out of the steam vessels or condensers by means of pumps wrought by the 

 engines themselves or otherwise. Fourthly ; I intend in many cases to employ the 

 expansive force (pressure) of steam to press on the pistons, or whatever may be 

 used instead of them, in the same manner as the pressure of the atmosphere is now 

 employed in common fire engines. In cases where cold water cannot be had in 

 plenty, the engines may be wrought by this force of steam only, by discharging the 

 steam into the open air after it has done its office. Fifthly ; where motions round 

 an axis are required, I make the steam vessels in form of hollow rings or circular 

 channels, with proper inlets and outlets for the steam, mounted on horizontal axles 

 like the wheels of a water mill ; within them are placed a number of valves that 

 suffer any body to go round the channel in one direction only : in these steam 

 vessels are placed weights so fitted to them, as entirely to fill up a part or portion 

 of their channels, yet rendered capable of moving freely in them by the means 

 hereinafter mentioned or specified. When the steam is admitted in these engines 

 between these weights and the valves, it acts equally on both, so as to raise the 

 weight to one side of the wheel, and by the re-action on the valves successively, 

 to give a circular motion to the wheel, the valves opening in the direction in which 

 the weights are pressed, but not in the contrary ; as the steam vessel moves round, 

 it is supplied with steam from the boiler ; and that which has performed its office 

 may either be discharged by means of condensers, or into the open air. Sixthly, 

 I intend in some cases to apply a degree of cold not capable of reducing the steam 

 to water, but of contracting it considerably, so that the engines shall be worked by 

 the alternate expansion and contraction of the steam. Lastly ; instead of using 

 water to render the piston or other parts of the engines air and steam-tight, I 

 employ oils, wax, resinous bodies, fat of animals, quicksilver, and other metals, in 

 their fluid state. 



" Be it remembered, that the said James Watt doth not intend that any thing in 

 the fourth article shall be understood to extend to any engine where the water to 

 be raised enters the steam vessel itself, or any vessel having an open communica- 

 tion with it." 1 



The great and valuable improvement described in this specification is that of 



1 Robisou's Mech. Phil. vol. ii. p. 119. Repertory of Arts, i. 217. 1794. 



