92 Lord Baijleigli [March 28, 



and give the wind, as it were, a better hold of them. It is only in 

 that way that large waves can be affected. The immediate effect is on 

 the small waves which conduce to that breaking of the large waves 

 which from the sailor's point of view is the worst danger. It is the 

 breaking waters which do the mischief, and these are quieted by the 

 action of the oil. 



I want to show also, though it can only be seen by those near, the 

 return of the oil when the wind is stopped. The oil is at present 

 driven to one end of the trough ; * when the wind stops it will come 

 back, because the oil film tends to spread itself uniformly over the 

 surface. As it comes back, there will be an advancing wave of oil ; 

 and as we light the surface very obliquely by the electric lamp, there is 

 visible on the bottom of the trough a white line, showing its progress. 



Now, as to the explanation. The first attempt on the right lines 

 was made by the Italian physicist, Marangoni. He drew attention 

 to the importance of contamination upon the surface of the water, and 

 to its tendency to spread itself uniformly, but for some reason which 

 I cannot understand, he applied the explanation wrongly. More 

 recently Eeynolds and Aitken have applied the same considerations 

 with better success. The state of the case seems to be this : — Let us 

 consider small waves as propagated over the surface of clean water ; 

 as the waves advance, the surface of the water has to submit to 

 periodic extensions and contractions. At the crest of a wave the 

 surface is compressed, while at the trough it is extended. As long as 

 the water is pure there is no force to oppose that, and the wave can 

 be propagated without difficulty ; but if the surface be contaminated, 

 the contamination strongly resists the alternate stretching and con- 

 traction. It tends always, on the contrary, to spread itself uniformly ; 

 and the result is that the water refuses to lend itself to the motion 

 which is required of it. The film of oil may be compared to an inex- 

 tensible membrane floating on the surface of the water, and hampering 

 its motion ; and under these conditions it is not possible for the 

 waves to be generated, unless the forces are very much greater than 

 usual. That is the explanation of the effect of oil in preventing the 

 formation of waves. 



The all-important fact is that the surface has its properties 

 changed, so that it refuses to submit to the necessary extensions and 

 contractions. We may illustrate this very simply by dusting the 

 surface of water with sulphur powder, only instead of dispersing the 

 sulphur, as before, by the addition of a drop of oil, we will operate 

 upon it by a gentle stream of wind projected downwards on the surface, 

 and of course spreading out radially from the point of impact. If Mr. 

 Gordon will blow gently on the surface in the middle of the dusty 

 region, a space is cleared ; f if he stops blowing, the dust comes back 



* May 1890. Any moderate quantity of oil may be driven off to leeward ; but 

 if oleate of soda be applied, the quietins: effect is permanent, 

 t This experiment is due to Mr, Aitken 



