i54 WELLS'S NATURAL PHILOSOPHY. 



How dees the 347. The resistance which a liquid opposes 

 S'l'to'a solid to a solid body moving through it, varies with 

 u vary?""'"""'' ^he form of the body. 



The resistance which a plane surface meets with while it 

 moves in a liquid, in a direction perpendicular to its plane, is in general, pro- 

 portioned to the square of its velocity. 



Wliat advan- ^^ ^^^^ surface of a solid moved against a liquid be presented 



tagebasanob- obliqufly with respect to the direction of its motion, instead 

 in^^^mo^^lfl''^ of perpendicularly, the resistance will be modified and diuiin- 

 a.^'ai>lst a liq- ished ; the quantity of liquid displaced will bo less, and the 

 surface, acting as a wedge, or inclined plane, will possess a 

 mechanical advantage, since in displacing the liquid it pushes it aside, instead 

 of driving it forward. 



The determination of the particular form which should be given to a mass 

 of matter in order that it may move through a hquid with the least resistance, 

 is a problem of great complexity and celebrity in the history of mathematics, 

 inasmuch as it is connected with nearly all improvements in navigation and 

 naval architecture. The principles involved in this problem require that the 

 length of a vessel should coincide with the direction of the motion imparted 

 to it ; and they also determine the shape of the prow and of the surfaces be- 

 neath the water. Boats which navigate still waters, and are not intended to 

 carry a great amount of freight, are so constructed that the part of the bot- 

 tom immersed moves against the liquid at a very obUque angle. 



Vessels built for speed should have the greatest possible length, with merely 

 the breadth necessary to stow the requisite cargo. 



The form and structure of the bodies of fishes in general, are such as to en- 

 able them to move through the water with the least resistance. 



_,_ ., 348. In the paddles of steamboats, that one is only com- 



Wnen are the ^ "^ 



paddles of a pletely effectual in propelling the vessel which is vertical in 



effecTivr?'^™''^* the water, because upon that one alone does the resistance 



of the water act at right angles, or to the best advantage. 



In the propulsion of steamboats, it is found that paddle-wheels of a given 



diameter act with the greatest effect when their immersion does not exceed 



the width, or depth, of the lowest paddle-board ; their effect also increases 



with the diameter of the wheel. 



J. The amount of power lost by the use of the paddle wheel 



wheel an ad- as a means of propelhng vessels is very great, since, in addi- 



vantafreous ^j ^^ ^^10 fact that onlv the paddle which is vertical in the 



motnod of ap- - '^ 



rlyinj? power water is fully effective, the scries of paddles in descending 



vessels"?^*'^^ ^^^ *^^ water, are obliged to exert a downward pressure, 



which is not available for propulsion, and in ascending, to hft 



a considerable weight of water that opposes the ascent, and adheres to the 



paddles. The rolling of the vessel, also, renders it impossible to maintain the 



paddles at the requisite degree of immersion necessary to give them their 



greatest efficiency ; one wheel on one side being occasionally immersed too 



