THE MOTION OF STEAM VESSELS. 313 



Additional Experiments. 



The Medea being moored in the basin at Woolwich, the throttle-valve of her en- 

 gines quite open, and one wheel only in action, the number of strokes was found to 

 be 12J per minute. Both wheels being now put in gear, the number of strokes 

 per minute was 8 J ; so that the resistance on a double surface, at a velocity of 8 J, is 

 equal to that on a single surface at a velocity of 12^ : from which it follows, (with a 

 very little allowance for the friction attending the working of the paddles,) that the 

 resistance, notwithstanding the violent commotion of the water, is very nearly as the 

 square of the velocity ; which law is therefore adopted in the following investi- 

 gations. 



It should be observed, that the engine was scarcely in perfect order when these 

 experiments were made ; but being in the same state in both cases, the results are 

 quite comparable. 



In an experiment on the Phoenix made subsequently to that in the Table, she was, 

 after being laden, lashed to the Warrior hulk, and her engines started. When her 

 wheels acted with the tide, the number of revolutions per minute was 7\, against 

 the tide 6 J, and when free 16 J, her speed then being 90 1 miles per hour. 



Illustrations of the action of Paddle-wheels in a Vessel in motion. 



In order to dispose of the power of an engine to the best advantage, it becomes 

 first necessary to know the manner in which it is at present consumed, and to cal- 

 culate accurately that portion which is effective in propelling the vessel. When this 

 is fairly understood, the arrangement and proportion of the parts which will effect an 

 improvement will be readily seen. 



When a steam-vessel is in motion, the force which opposes the engine is the 

 resistance produced by the paddles moving through the water at a velocity equal to 

 the difference of that of the centre of pressure of the wheel and that of the vessel. 

 The part of this resistance which, when resolved, is in a horizontal direction, is that 

 which is effective : the remaining part of the power is consumed by the resistance 

 opposed to the paddles in a vertical direction, the back water, and other circumstances 

 attending this mode of exerting the power of an engine. Some additional velocity may 

 be given by the tendency of the paddle in its descent to raise the vessel in the water, 

 by diminishing the sectional area of resistance : this, however, if any, is so small as not 

 to be worth consideration ; and it may therefore be fairly assumed that the horizontal 

 resistance above mentioned is equal to that opposed to the motion of the vessel. 



To make a calculation of these resistances, it becomes necessary to find, with some 

 degree of accuracy, the position of the centre of pressure in the float or paddle, as 

 the calculation is built upon the difference of the velocities of the boat and this cen- 

 tre, which are in some cases so nearly equal that the top of the paddle has no motion 

 through the water. 



