STEAM NAVIGATION. 



under such, conditions the screw is less efficient than the paddle ; 

 for though both vessels attained the same speed of four knots 

 against a strong head wind, yet, in the case of the " Alecto," this 

 performance was attained with a velocity of the engine of 12 

 strokes per minute, whereas in the " Rattier" it was only attained 

 with a velocity of the engine of 22 strokes per minute. It follows, 

 therefore, that a screw-vessel in proceeding head to wind will 

 require 1*8 times, or nearly twice the quantity of fuel to do the 

 same amount of work? The screw, in fact, revolves at nearly the 

 same velocity whether the wind is adverse or favourable, or 

 whether the vessel is lying at anchor ; and this is a serious defect 

 in the case of vessels intended to encounter adverse winds. In the 

 case of vessels, however, which use the screw only as a resource in 

 calms, or as an auxiliary to the sails, this disadvantage will not be 

 experienced, since such vessels have no pretensions to the capability 

 of proceeding in direct opposition to a strong head wind. 



74. Among the experiments made with the "Alecto" and 

 1 1 Rattler," some of the most interesting and important were 

 directed to the determination of the relative towing powers of the 

 screw and paddle-wheel. For this purpose the two ships were 

 lashed stern to stern, and the engines of both were set to work so 

 as to make them draw the connecting chain in opposite directions. 

 In these and all other cases where screw and paddle-vessels of 

 equal power and size have been thus connected, the screw-vessel 

 has preponderated, and towed the paddle-vessel as soon as the 

 engines were set to work. 



When the "Rattler" and "Alecto" were lashed together in this 

 manner, the " Alecto' s " engines were set on first, and she was 

 allowed to tow the " Rattler" at the rate of two knots an hour. 

 The " Rattler's " engines were then set on. In five minutes the 

 two vessels became completely stationary. The "Rattler" then 

 began to move ahead, and towed the " Alecto " against the whole 

 force of her engines, at the rate of 2-8 knots per hour. In like 

 manner the " Niger " towed the " Basilisk " astern, in opposition 

 to the force of her engines at the rate of 1 % 1 knots per hour. The 

 natural inference from this experiment would be that the screw is 

 more suitable for towing than the paddle ; yet this inference is not 

 confirmed by the experiment, for when the " Niger " and 

 "Basilisk" were each set to tow the other alternately, in the 

 usual manner in which a steamer tows a ship, it was found that 

 the " Niger " towed the " Basilisk " at a speed of 5-63 knots, with 

 593-9 horse-power, and that the "Basilisk" towed the "Niger" 

 at the rate of 6 knots, with 572-3 horse-power. The paddle- 

 vessel, therefore, accomplished in towing the largest speed with 

 the least power. It has also been found that when a paddle 

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