March 5, 1886.] 



SCIENCE. 



215 



1837, band vi. p. 1750 ff.) there is a good presenta- 

 tion of the subject, and many facts are cited. Most 

 of them are drawn, however, from the thirty-page 

 discussion in Weber's ' Wellenlehre ' (1825). Here 

 one finds quotations from Aristotle, Plutarch, and 

 Pliny, which show that in early times the power of 

 oil to quiet, and so render more transparent, the sur- 

 face of water, was known. Eeferences are made to 

 other and later writers, and to the facts collected by 

 Franklin ; and details are given of experiments made 

 by him (Phil, trans., Ixiv., 1774), and later by the 

 Webers. 



From all these data, as well as from the recent 

 observations reported in Science (especially vii. p. 

 134), it seems that the effect of the oil-film is to 

 diminish the ' combing ' of the waves, and to prevent, 

 in part at least, the formation of small waves, and 

 the growth and sharpening of the crests of the large 

 ones by the continued action of the wind. The 

 exaggerated popular notion that the great waves are 

 quieted seems to be erroneous. The only known 

 ways of destroying in the open sea the energy of a 

 wave once formed are by fluid friction, by rain, and 

 by an opposing wind. But we must not under- 

 estimate the advantage of preventing the piling-up, 

 on a wave already dangerously high, of another only 

 a few inches high. On the well-known principle of 

 superposition, it must sometimes happen that the 

 crests of waves belonging to two or more systems 

 •will coincide. The resultant wave is then higher, 

 and exposes more surface to the wind : and the crest, 

 being sharper, is more easily blown off by the wind : 

 so, as the wave is likely to run faster than the ship, 

 it may break over her in a way that would not hap- 

 pen if it were only a little lower, — if only one of its 

 smaller components could be suppressed. " 



It is further to be borne in mind, in seeking an 

 explanation for the indisputable and useful effect of 

 oil, that, as the passage of a wave is the transfer of 

 energy but not of matter, the oil will not be carried 

 onward by the wave ; and that, if the formation of 

 new waves over a given large surface could be pre- 

 vented, the old ones would speedily pass out of it, 

 and those coming into this surface from beyond 

 would not be increased, but would decrease some- 

 what, because of the fluid friction. 



The practical problem, therefore, before the ship- 

 master, is to find some means, 1°, of preventing the 

 formation of new waves, or the growth of old ones, 

 over a given surface to the windward of his ship, 

 and, 2°, of making this surface as large as possible. 

 He solves it more or less completely by the use of 

 oil ; and now we seek an explanation of the action 

 of oil from the physicist. 



The German physicists of the first part of this 

 century followed pretty generally the view attributed 

 originally to Aristotle, and elaborated by Franklin ; 

 the Webers subscribe to it ; and Miincke, in 4 Gehler,' 

 says it is generally held : in a word, the friction of 

 the wind is less on the oil than on the water. Stated 

 in this way, however, the sentence is almost sure to 

 convey a false impression. We know of absolutely 

 no proof that this is true, if taken with its obvious 

 meaning ; but the truth it embodies is simply, that, 

 owing to the interposition of the oil-film, the force 

 of the wind is not communicated to the water ; and 

 this can be explained in a way consistent with mod- 

 ern physical notions. Franklin had pointed out how 

 a ripple raised by the wind gets higher, broader, and 

 longer at each successive vibration [and therefore 



travels faster] : he compares the effect of the wind 

 to the setting of a heavy church-bell to swinging, by 

 properly timed impulses of a finger. He thinks the 

 adhesion between the oil and water is so slight (if, 

 indeed, the repulsion be not strong enough to main- 

 tain the film at a small distance from the water) that 

 the film can be moved a little by the wind without 

 disturbing the water. He suggests, further, that 

 the wind can ' catch ' hold of the large wave better 

 when this is covered with ripples, while, if it be oiled, 

 the wind may press it down. The Webers add some- 

 thing with reference to the resolution of the force of 

 the wind, which seems not quite sound in theory ; 

 and Miincke has something to say about a slight bind- 

 ing of the surface of the water by the oil. 



But some properties of fluids unknown to the 

 earlier physicists have a bearing on the present prob- 

 lem. Thus Daniell, in his ' Principles of physics ' (p. 

 247), says, under the title ' Superficial viscosity,' " To 

 the same cause [superficial tenacity] we must at- 

 tribute the smoothing of the surface of a rough sea 

 when oil is poured upon it : the new surface has 

 great superficial tenacity and small superficial ten- 

 sion, and is not readily broken up into surf." The 

 bearing of this may be shown thus. Imagine a per- 

 fectly calm lake : a wind strikes it, and it is covered 

 with wavelets. It is not the increase of pressure 

 over the lake that causes the waves, but slight differ- 

 ences of pressure between neighboring points, due to 

 the fact that the winds flow more or less in gusts, 

 not steadily. If the surface were solid, or very vis- 

 cous, like mucilage or thick oil, the momentary force 

 due to the difference of pressure would cease to act 

 before any sensible movement could take place. The 

 effect would be the same in kind, though differing in 

 amount, however thin the film, or slightly viscous the 

 oil may be ; but we should remember that the super- 

 ficial viscosity which is effective here is usually 

 greater than the viscosity calculated from experi- 

 ments where a considerable volume of the liquid is 

 used. The effect, too, would be the same in kind, 

 though the sea were rough instead of calm. We see, 

 then, that an oil- film, by its viscosity (as well as by 

 slipping over the water, if Franklin's view is cor- 

 rect), delays the action of the wind's force on the 

 water for so long a time, that the force may have 

 ceased to act before any movement begins, and then 

 no work is done by the wind on the water. Thus, in 

 an extreme case, no new waves are formed, and those 

 driven on by the wind through the oil-covered sur- 

 face do not have their crests continually elevated and 

 sharpened till they are ready to break. 



What might happen in an extreme case does hap- 

 pen, to some extent, in every case where oil is used 

 on the water. Thus the wake of a ship generally 

 shows a surface covered with bubbles more persistent 

 than usual, and comparatively free from small waves, 

 both effects being probably due to the traces of oil 

 coming from bilge-water, the cook's galley, etc. 

 Where a ship is driven before the wind, and the 

 waves are running faster than the ship, if oil is being 

 used, it is evident that the wind has to pass over a 

 long oil- covered surface, and the effect of the oil 

 will be especially favorable. Since it is essential to 

 this explanation that the oil be spread to the wind- 

 ward, little benefit is to be expected from the use of 

 oil on waves coming from a distant storm ; nor when 

 the wind is ahead, unless means can be used to throw 

 the oil a long distance ahead. 



If this explanation be correct, as we believe it to 



