ON THE HISTORY OF HYDRAULICS AKD PNEUMATICS. 363 



traction of the steam. Lastly, instead of using water to render the piston or 

 other parts of the engines air and steam tiglit, 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 diflficulty 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 of 

 parliament until the year 1799 ; but although the legal privilege of the ori- 

 ginal manufacturers is expired, yet the superiority of their workmanship still 

 gives thfeir engines a decided preference. 



Much of the labour of the later writers on hydraulics has been employed 

 en the determination of the resistance of fluids to bodies of different forms, 

 which move through them; a subject which derives great importance from 

 its immediate application to the manoeuvres of ships. The most ex- 

 tensive experiments on these subjects were made by Bossut, and 

 some other members of the Academy of Sciences. About the same time Don 

 George Juan, a gentleman who had enjoyed the best possible opportunity 

 for actual observation and practical study in serving with Ulloa, published 

 at Madrid his Examen Maritime, which appears to be the most ingenious 

 and useful treatise on the theory and practice of seamanship that has yet 

 appeared. But unfortunately his deductions, however refined and diversi- 

 fied, are principally founded on a mistaken theory respecting the effects of 

 hydraulic pressure; since he tacitly assumes, in his fundamental proposition 

 on the subject, that a double force, acting in a given small space, will pro- 

 duce a double velocity; while it is well known that in such clrcuinstances 

 a quadruple force would be required. Hence he derives some conclusions 

 which indicate that the resistance must vary very materially at different 

 depths below the surface of the water, and alleges In support of the assertion 

 a few imperfect experiments of Mariotte and of his own, in which some acci- 

 dental circumstances not noticed may easily have caused great irregularities. 

 Mr. Prony, in his Architecture Hydraulique, appears to have followed Juan ; 

 and Professor Robison very justly observes, in speaking of this work, that 

 if the pressure of the water alters the magnitude of tlve resistance at different 



