MECHANICS AND USEFUL ARTS. 29 



course assisting, till it reaches the surface. It is then either pumped out, 

 careened, made tight and set afloat, or held up and towed into ten or twelve 

 feet of water, or if it weigh less than 1000 tons, is deposited on the doclv. 



The measurement of the great derrick is 5000 tons. Its cost was $250,000 

 equal to that of a first-class Sound or North River steamer. 



The steamer Ericsson, of 2200 tons, which, it will be remembered, sunk off 

 the Jersey coast some years since, was lifted to the surface of the water by a 

 derrick of only 300 tons capacity. But the great derrick, with a capacity of 

 ] 000 tons, will raise to the surface the largest ship ever sunk, pump it out at 

 the rate of 60 tons of water a minute, and hold it fast till it is either put into 

 floating condition, or stripped of its valuables. All it requires is a good hold, 

 and this it can get, deeper than divers ever sounded. As to ordinary vessels 

 of 200 or 400 tons, whose actual weight would be within the capacity of 

 the great lifter, it would simply pick them up from the bottom, carry them 

 ashore, and land them high and dry on the dock. 



EXPERIMENTS WITH SCREW-PROPELLERS. 



A series of experiments with screw-propellers has recently been made 

 in England, with a view merely of testing the relative qualities of the com- 

 mon screw with Griffith's propeller. The common screw used by the British 

 Admiralty, consists of a sixth part of the whole helix; Griffith's propeller 

 has a spherical central base, one-third the diameter of the screw, with the 

 blades made tapering. The driving surface of the former is at the extreme 

 ends of the blades ; that of the latter lies towards the centre, nearest the 

 sphere. 



The first trial was with a common screw, which had a diameter of 18 feet; 

 the speed obtained was 11'823 knots per hour. On a second trial, with its 

 diameter increased to 20 feet, the speed. was 11 '826 knots; but there was a 

 gi-eat increase of vibration. The leading corner of each blade was now cut 

 off, and on the third trial, with this change, a speed of 12'032 knots was ob- 

 tained. Both corners of the blades were now cut off, and a fourth trial made; 

 but even with a greater number of revolutions, less speed 12'012 knots 

 was secured. The highest speed was therefore achieved with the leading 

 corner of the screw cut off. With a Griffith's propeller of 20 feet diameter, 

 and 32 feet pitch, the first trial gave 1T981 knots per hour; on a second 

 trial, with an alteration of pitch to 26 feet 5 inches, a speed of 12'269 knots 

 was the result; on a third trial, with a still further reduced pitch, and 43| 

 revolutions per minute, there was much less vibration than on former trials, 

 but the speed was only 12' 158 knots. 



It was found during these trials that the leading edge of the screw is the 

 part which affects the steering of a vessel most, and causes the greater part 

 of the vibratory action. It was also demonstrated that an increased diam- 

 eter of the common screw was better than an increased pitch to reduce, the 

 speed of the engines, with an augmented speed in the vessel ; but it had the 

 effect of promoting the vibration, which is an evil to be avoided, if possible. 

 By increasing the diameter of the Griffith's propeller, additional vibration 

 was not experienced, because its chief acting surface is not at the extremities 

 of the blades. These experiments seem to have established the fact, that a 

 propeller having a sphere at its central portion, combined with tapering 

 biades, gives better results with less power than the common screw-pro- 

 peller. 



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