ON THE HISTORY OF HYDRAULICS, &c. 283 



steam that enters it ; first, by inclosing it in a case of wood, or any other 

 materials that transmit heat slowly ; secondly, by surrounding it with 

 steam or other heated bodies ; and thirdly, by suffering neither water, nor 

 any other substance colder than the steam, to enter or touch it during that 

 time. Secondly, in engines that are to be worked wholly or partially by 

 condensation of steam, the 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 neigh- 

 bourhood 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 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 capable of moving freely in them by 

 the means herein after mentioned or specified. When the steam is admitted 

 in these engines between the weights and the valves, it acts equally on both, 

 so as to raise the weight to one side of the wheel, and, by the reaction of 

 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 may 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." 



It is probable that the rotatory engines described by Mr. Watt, although 

 they appear to produce some advantage in theory, will never be generally 

 introduced, on account of the difficulty of constructing steam vessels so 

 large, and of so complicated a form, as would be necessary, in order to give 

 full effect to the machine. The term of this patent was prolonged by act 

 o parliament until the year 1799 ; but although the legal privilege of the 

 original manufacturers is expired, yet the superiority of their workmanship 

 still gives their engines a decided preference. 



