284 LECTURE XXX. 



Much of the labour of the later writers on hydraulics has been em- 

 ployed on 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 

 extensive 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 Maritimo, which appears to be the most ingenious 

 and useful treatise on the theory and practice of seamanship that ha 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 pro- 

 position on the subject, that a double force, acting in a given small space, 

 will produce a double velocity ; while it is well known that in such cir- 

 cumstances 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 accidental 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 magni- 

 tude of the resistance at different depths, that of the atmosphere ought by 

 no means to be omitted in the calculation. But if a more correct mathe- 

 matician and mechanic would take the pains to model Juan's book anew, 

 to correct his errors, and to adapt his modes of calculation to the laws of 

 resistance previously deduced from accurate experiments rather than from 

 theory, there is no doubt but that the work thus modified might essentially 

 improve the science of seamanship. He alleges indeed that the results of his 

 calculations are in almost every instance rigidly conformable to observa- 

 tion and experience, but it is probable that where such a coincidence really 

 exists, it must be owing to some combination of errors compensating each 

 other ; and it is indeed very possible that his calculations, with all their 

 errors, may approach nearer to the truth than the imperfect approximations 

 which had been before employed. Juan has generally made use of the 

 English weights and measures, on account of their convenience in compu- 

 tations respecting the descent of falling bodies and the impulse of water. 



The works of Chapman and of Romme, upon various departments of 

 seamanship, possess also considerable merit. These authors appear to 

 have avoided the errors of Juan, but without entering so minutely into the 

 detail of nautical operations as he has done. 



The accurate experiments of Dr. Hutton and of Count Rumford on the 

 force of fired gunpowder and the resistance of the air, deserve to be men- 

 tioned as affording valuable materials to the speculative investigator, and 

 useful information to the practical gunner. Robins had very erroneously 

 supposed that the whole of the effects of gunpowder might be derived 

 from the expansive force of fluids permanently elastic ; but Vandelli soon 



