TRANSACTIONS OF THE SECTIONS. 221 



MECHANICAL SCIENCE. 



Address hy Wii-liam Froude, Esq., C.E., 3I.A., F.B.S., President of 



the Section, 



Thk addreas of the President of a Section would year by year possess an appro- 

 priate interest, if it could always consist of an exposition of the progress made dui-ing 

 the past year in the department of science which the Section embraces. And 

 many of the addresses to this and other sections have conformed to this pattern 

 with marlicd success. 



But the adequate preparation of an address shaped in this approved mould 

 would require a range of experience and a grasp of thought such as few possess ; 

 and custom has wisely sanctioned a type of address which, though less appropriate 

 to the occasion, need not be either uninteresting or inapposite. And we, in this 

 Section, have not to search far for instances in which its President has charmed 

 and instructed us by a masterful exposition of some single subject in practical 

 science, or by a timely reminder of the improvident manner in which we deal with 

 some precious store of natural wealth. 



I must express a hope that it will not be regarded as a conversion of liberty into 

 licence, if the subject I have chosen obliges me to introduce a further innovation, 

 and to use diagrams and experiments in order to make my meaning clear. 



I propose to treat of certain of the fundamental principles which govern the be- 

 haviour of fluid, and this with special reference to the resistance of ships. By the 

 term "resistance" I mean the opposing force which a ship experiences in its pro- 

 gress through the water. 



Considering the immense aggregate amount of power expended in the propulsion 

 of ships, or, in other words, in overcoming the resistance of ships, I trust you will 

 look favourably on an attempt to elucidate the causes of this resistance. It is true 

 that improved results in ship-building have been obtained tlirough accumulated 

 experience ; but it unfortunately happens that many of the theories by which this 

 experience is commonly interpreted are interwo^■eu with fimdameutal fallacies, 

 which, passing for principles, lead to mischievous results when again applied 

 beyond the limits of actual experience. 



The resistance experienced by ships is but a branch of the general question of tlie 

 forces which act on a body moving through a fluid, and has within a compara- 

 tively recent period been placed in an entirely new light by what is commonly 

 called the theory of stream-lines. 



The theory as a whole involves mathematics of the highest order, reaching alike 

 beyond my ken and my purpose ; but I believe that, so far as it concerns the resist- 

 ance of ships, it can be sufficientlj^ understood without the help of technical mathe- 

 matics ; and I will endeavour to explain the course which I have myself found 

 most conducive to its easy apprehension. 



It is convenient to consider tirst the case of a completely submerged body moving 

 in a straight line with imiform speed through an unlimited ocean of fluid. A fish 

 in deep water, a submarine motive torpedo, a sharp-ended sounding-lead while 

 descending through the water, if moving at imiform speed, are all examples of the 

 case I am dealing with. 



It is a common but erroneous belief that the resistance to its onward motion ex- 

 perienced by such a body thus moving, originates in an increase of pressure through- 

 out its head end, and a diminution of pressure throughout its tail end. It is thus 

 supposed that the entire head end of the body has to keep on exerting pressure to 

 drive the fluid out of the way, to force a passage for the body, and that the entire 

 tail end has to keep on exerting a kind of suction on the fluid to induce it to close 

 in again — that there is, in foct, what is termed plus pressure throughout the head 

 end of the body and minus pressure or partial vacuum throughout the tail end. 



This is not so ; the resistance to the progress of the body is not due to these 

 causes. The theory of stream-lines discloses to us the startling but true propo- 

 sition, that a submerged body, if moving at a uniform speed tlu'ough a perfect 



1875. 17 



