8 Some Experiments in Propulsion. 



occurred the machine worked with great vigour, the screw 

 sometimes making as many as 180 revolutions per minute. 

 It should be remembered, however, that this great speed of 

 rotation of the screw is not the best suited for propul- 

 sion, on account of the creation of what is known as negative 

 slip of the screw. Indeed the difficulty throughout in the 

 experiments which have been made is not in obtaining 

 sufficient power, but rather in controlling the excess of it. 

 The wind on the occasion under notice was off shore, the 

 waves therefore very small, about four feet long, and a few 

 inches only in height, with a period of six seconds. The 

 reason why the period of the waves is so important an 

 element in the effect produced, is that the efficacy of the 

 principle depends mainly on the velocity of the movements, 

 not their magnitude, as shown in the fact that the model in 

 question worked vigorously with the movement of only an 

 inch, repeated however ten times per minute. In point of 

 theory the action of the apparatus involves some very 

 abstruse points; indeed it had proved not a little perplexing 

 to those who had witnessed it. Mr. Froude, at the first 

 meeting of the Institution of Naval Architects, 1874, referred 

 to the principles involved in the action of the machine as a 

 very obscure subject ; and again, at the British Association 

 at Bristol, September, 1875, he spoke of it as a most complex 

 proposition which he and others had at first only dimly seen 

 through. Mr. G. Rendel also, the distinguished engineer 

 and originator of the " Staunch" class of gun-boats, and the 

 partner of Sir Wm. Armstrong, has referred to the principle 

 of the machine as (to repeat Mr. Rendel's words) a very 

 curious and beautiful idea, and that it has been well worked 

 out ; as a scientific principle, he adds, he considers it perfect. 

 Similarly at the April session of the Institution of Naval 

 Architects, Lord Hampton^ the President of the Institution, 

 spoke of it as one of the most important, but at the same 

 time most difficult, of projects. It need hardly be added 

 that the development of a principle so little understood as 

 is admitted in these opinions is necessarily a work of slow 

 progress, when every step in the demonstration nearly ex- 

 hausts for a long time individual means. 



The dynamical effect exhibited by the model during the 

 experiments as accurately taken at the time, was at the rate 

 of IJ horse-power per ton of working load. With regard to 

 this vigour of action, which occasioned some surprise at the 



