46 YEAK-BOOK OF FACTS. 



THE SCREW- PKOPELLER. 



Her Majesty's screw vessel Cygnet has completed at Portsmouth 

 a series of highly interesting and important experimental trials with 

 the Griffiths Propeller, testing some suggested improvements in its 

 form by the patentee. It has always been a great object with* 

 engineers to obtain greater propelling power from the screw, without, 

 at the same time, increasing its size, in order that screw ships with full 

 power might be constructed with a light draught of water, and at 

 the same time have the screw well immersed. The present trial in 

 the Cygnet is the nearest approach to this ye tattained ; and a f;dr 

 hope may be now entertained that, after having puzzled scientific 

 men during the past twenty years, the marine screw-propeller may 

 for the future be more fully understood. 



It is now upwards of ten years since Mr. Griffiths brought forward 

 his theory of the screw — viz., that the centre part of the screw, equal 

 to one-third its diameter, should be filled up, and the screw-blades 

 should be wide at the root, and tapering towards their extremities, in 

 contradistinction to the generally-received rule, and the form of the 

 Admiralty pattern. 



The improvements suggested by Mr. Griffiths in the form of his own screw, 

 and which have now been tested in the Cygnet with the greatest success, consist 

 of an ad Jit inn of an angular surface at the after edges of the screw blades, 

 springing from their widest part, and increasing the width of the angulated por- 

 tion as it proceeds outwards to the periphery or ciroumfi blade. 

 These angular surfaces stand at an inclination to the after-face of the blades : 

 consequently, as they rotate, the water, which has been acted on and put in 

 motion by the fore part of the blade, is again struck by this after, or angulated 

 portion, thus making the blades double-acting j whereby, it is stated by Mr. 



DS, smaller diameter of screw-propeller may be used without deci 

 the power employed or given by the larger diameter in the old form ; the theory 

 of this being that the front of the blade navels at the rate of 3000ft. per minute, 

 while the root, or wider portion of the blade, only moves at the rate of 1600. 

 The water, being struck with the higher rate of velocity at the point of the blade, 

 recedes offj and the after, or angulated portion, catching the Bame body of water, 

 icond time. In the lirst trials the C*gnaf$ screw was 9ft. diameter, 

 l L2ft. pitch, the engines in this instance making lit revolutions per minute. 

 With the same screw reduced to Tit. (tin. diameter, and 12ft. pitch, with the 

 angulate 1 addition made to the blades, the revolutions of the i 

 while the peed of the ship was the same on both trials, the 7ft 6in. screw thus 

 doing an equal amount of work with the 8ft, BOMW, and « ilutionB. 



The — , erwarda reduced to a 7ft. diameter, with the sat c pitch as 



The result gave 111 » ter knot 



i. The Bternposta of t] ■• IS inches in width, and, as a 



matter of COUTfle, produced a greater deteriorating influence Up u the small 

 tponthe larger one. The sphere in the smaller screw WW 

 as in the larger one. Both these causes, therefore, militated much 



orew, notwithstanding its success, than ia shown l>y 

 ■ . that the reci pi trial of the IV. neb authorities 

 in thi^ir endeavours to obtain the tame results which I ave now been achieved by 

 these trials, were conducted ontl pi indole, and have t. 



THE ABMBTRONG ami WHTTWOBTH BXTLKD CANS 



Fob (bur weeks in succession, cadi Tuesday night, the Institution 



nf Civil Engineers, €lreat G< . \'> itminster, was crowded 



by mi bear the papers and di d Rifled Cannon — 



Armstrong and fcheWhitworth Killed Cannon— actual working 



samples of these rivals (twelve-pounders) having been plac< '1 on the 



