January 23, 1886.] 



SCIENCE. 



77 



trary sides, there is no tendency to mix together ; but 

 the division between the two, called the k cold wall,' 

 is nearly a vertical plane. This is the whole mystery 

 of the Gulf Stream and of the cold wall. 



The level of the Mediterranean Sea at Marseilles is 

 undoubtedly a little lower than that of the Strait of 

 Gibraltar and of the ocean generally adjacent to the 

 north-western coast of Africa ; so that the latter is 

 about on a level with the western extreme of the Gulf 

 of Mexico, there being a little down-grade across to 

 the West Indies, and then a little ascending grade to 

 the coast of Mexico to check the westward motion, 

 and to deflect the current around toward the north. 

 The difference, therefore, between the ocean-level at 

 New York and Brest is probably about five feet. 



There is another theory, the wind-theory, which is 

 thought by some to explain satisfactorily all the cur- 

 rents of the ocean. It may be well to examine a 

 little here the claims of this theory, and especially to 

 consider whether it is adequate to explain 'the 

 recently observed differences of sea-level. The west- 

 ward component of the trade-winds, by this theory, 

 raises the level of the Gulf of Mexico, and depresses 

 the sea-level on the north-west coast of Africa as 

 much : and the eastward and north-eastward motion 

 of the air in the middle latitudes drives the water 

 toward the coast of Europe, and so causes a depres- 

 sion of the sea-level on the American coast, and a 

 raising of it on the coast of Europe. It is readily 

 seen that this would give precisely the same system 

 of circulation, and tend to cause the same differences 

 of level between the Gulf of Mexico and New York 

 harbor, and between the harbors of Brest and Mar- 

 seilles, as the other theory. But it is well known 

 that ordinary winds 1 have very little effect in chan- 

 ging sea-level, except in very shallow water. 



According to the Report of the chief of engineers 

 (1876, part iii. p. 76), by the mean of all observations, 

 the difference of mean level of Lake Ontario, at 

 either end, with north-east, east, and south-east 

 winds, and with south-west, west, and north-west 

 winds, is only 0.05 of a foot, and hence the average 

 effect of either class of winds on the surface level is 

 less than one-third of an inch. 



Again, if the trade-winds cause a raising of the 

 sea-level in the Gulf of Mexico by a half-metre, they 

 must depress the sea-level on the Pacific coast of 

 Mexico about the same amount, and so there would 

 be a difference of level of about one metre on the two 

 sides. But by the levellings for the Nicaragua ship- 

 canal, the elevation of the surface of Lake Nicaragua 

 was found to be exactly 107 feet above mean tide of 

 either ocean. Hence the trade- winds have no sensible 

 effect in changing sea-level. 



Furthermore, if the trade- winds can have so great 

 an effect as is claimed for them, then the still stronger 

 westerly winds, which usually prevail in the middle 

 latitudes of the North Atlantic, should change the 

 difference of sea-level between New York and Brest 

 at least as much ; and if so, there would have to be a 

 considerable annual inequality in the height of sea- 

 level ; for the westerly winds are much more prev- 

 alent, and blow very much more strongly during the 

 winter than during the summer season, there ought, 

 therefore, to be a change of the height of sea level of 

 more than one foot, higher in winter and lower in 

 summer, on the east side of the Atlantic, and the 

 reverse on the other. But no such inequality is 

 observed on either side. Mean sea level is two or 

 more inches higher, on both sides, in summer than in 



winter, which is evidently due to the difference of 

 temperature of the sea-water in the two seasons, and 

 there is no apparent effect whatever arising from an 

 increase of the strength of the winds. The only 

 inference from this is that the strongest winds have 

 no sensible effect. 



A continuous wind, for some time in any direction, 

 evident^ causes mere surface currents of considerable 

 velocity ; but if they could even explain the strong 

 and deep flowing currents, such as the Gulf Stream, 

 it is evident, from what is shown above, that they 

 canuot account for the great differences of sea-level 

 which have been shown to exist by recent levellings. 



Wm. Ferrel. 



Washington, Jan. 18. 



Oil on troubled waters. 



I do not know much about the sea, and so perhaps 

 you will wonder the less at my expressing incredulity 

 with reference to the reports of the extraordinary 

 effect of ' oil on troubled waters,' to which you seem 

 to give unqualified assent in your notes and com- 

 ments of Jan. 15. 



It is indeed remarkable that seamen should have 

 overlooked this important aid to navigation, if, as 

 you declare, its efficiency in calming the waves is as 

 obvious as the use of the rudder in shaping a new 

 course ; for sailors are not usually slow to adopt 

 notions favorable to the existence of prodigies and 

 marvels. 



But, if the newspaper accounts of the matter are 

 to be believed, it strikes me that the hydrographic 

 office has quite outdone every other politico-scientific 

 bureau in the propagation of startling generalizations 

 from veiy flimsy details. For example : one of its 

 witnesses testifies that in 1863, when off Sydney 

 Head, he encountered a terrific gale, followed by a 

 tremendous sea, in which his ship was making water, 

 and was in danger of wreck, and that he at first tried 

 oil upon the waves by ' jerking it out ' over the side 

 of the vessel, through a hole in the cork of a bottle ; 

 but finding that when employed in this way it blew 

 about the stem of the ship, and not into the sea, he 

 made use of 'the oil-bag,' into which he put about 

 half a gallon, tying the neck tight, and towing it 

 astern. After a short time, he says, " the effect was 

 wonderful ; for what was a very heavy-running and 

 dangerous sea was reduced, by the use of the oil, into 

 what a seaman would call ' blind rollers,' quite harm- 

 less to a ship." He asserts that in this manner he 

 ran his half-sinking vessel from Sydney Head to Port 

 Stephens, a distance of sixty-eight miles, in eight 

 hours and a half, on a consumption of two gallons 

 and a half of oil, although he considers that his way 

 of using it was wasteful. His subsequent experiences 

 convinced him that a ship could run in any sea with 

 safety for twenty -four hours on a consumption of 

 five gallons of oil. 



It is hardly surprising, that, as soon as the hydro- 

 graphic office began spreading such sailors' yarns as 

 this, ether captains should have felt the necessity of 

 keeping abreast of the times in nautical science by 

 publishing their similar experiences. Accordingly the 

 skipper of the schooner J. B. Atkinson announces, 

 that, on the 25th and 26th of December, his vessel 

 was saved from utter destruction off Cape Hatteras 

 by bags of oil, which he also towed astern ; and still 

 later, the captain of the steamer Lucy P. Miller, 

 running between Philadelphia and Nassau, writes to 



