72 



NATURE 



\Nov. 23, 1871 



of his researches. In 1859 Bazin undertook similar experiments 

 with a small rectangular conduit ; but he did not make so great 

 a number of experiments, and his errors of observation are larger 

 than those of Darcy. In determining the coefficients of resist- 

 ance of these conduits, it was found that for those of Darcy 

 m = '0104, while for those of Bazin it rose to O'OlSo. 



Bazin performed a considerable number of experiments on the 

 motion of water in open conduits, and thought himself com- 

 pelled to admit that the laws of this motion are essentially diffe- 

 rent from those which relate to perfectly full conduits ; but he is 

 certainly in error. 



The results of a considerable number of ancient measures of 

 currents are in existence, which Bruning undertook, towards the 

 close of last century, in different rivers— namely, the Rhine, 

 Waal, &c. They were performed with much care ; but, as 

 might be foreseen, are nevertheless very defective. They deserve, 

 however, to attract attention, partly because the rapidity was 

 determined, for every section of the current, at distances of six 

 inches from the surface to the bottom, in a series of perpendi- 

 culars, the imperfections which the measurement of the rapidity 

 presents losing thus much of their importance ; partly and espe- 

 cially because the currents examined by Bruning were of a depth 

 which reached 23 feet. In applying this theory to these currents, 

 and especially in determining the constants of the formulce with 

 the aid of Bruning's measurements, it was found that the observed 

 and calculated rapidities are for all depths as accordant as could 

 be desired ; and this agreement furnishes a new proof of the 

 exactness of the theory. The coefficient of resistance m, calcu- 

 lated according to Bruning's measurements, varies between 

 0'0250 and 00080, with a mean of o 0160 ; and as the resistance 

 at the depth of these currents must doubtless approach that which 

 a marine current experiences in flowing over a mass of water 

 placed beneath it, and which docs not participate in the motion, 

 1 have a right to believe that the extreme value ;^/=oo25o corre- 

 sponds nearly to the resistance which currents meet with when 

 flowing freely in the sea. 



After having in this manner assured myself that the preceding 

 theory agrees with experiment wherever it has been tried, I 

 endeavoured to determine the laws of the motion of water in 

 currents of variable rapidity. In considering the simplest case of 

 this kind, that, namely, in which the conduit is a level surface (I 

 had already treated this case by the old theory), it was found 

 that the laws of currents, according to the new theory, are en- 

 tirely in agreement with the facts observed in nature ; and con- 

 sequently this theory may be regarded as giving the explanation 

 of all permanent currents. 



Having thus shown that this theory of the movement of fluid 

 bodies accounts satisfactorily for all the phenomena, I .shall now, 

 from this as a stand-point, give a review of my recent researches 

 upon ocean currents. The currents which more particularly de- 

 mand our attention here are those of the North Atlantic, es- 

 pecially the Gulf Stream and the Polar Currents. 



The Gulf Stream issues, as we know, from the Gulf of Mexico, 

 but it is possible to follow its course across the Carribean Sea, 

 where passing between the Antilles, it arriv'es from the Atlantic, 

 and afterwards flows to the north-west at the rate of ^ mile an 

 hour until it enters the Gulf of Mexico. From this Gulf the 

 Gulf Stream takes an easterly course towards the Bahamas along 

 the north coast of Cuba ; but, after rounding Florida, it bends 

 northwards, and passes between the latter and the Bahamas, in 

 the channel which separates Florida Cape from the Islets of 

 Bemini ; here ttie current has a speed of i mile per hour, a 

 breadth of 8 miles, and a depth of 250 fathoms. From the channel 

 of Bemini the Gulf Stream proceeds directly northwards at a rate 

 ■which decreases gradually from 6^ feet per second at Bemini to 

 4 feet at St Aug^istine ; the distance between these two points 

 being about 70 miles, during which the breadth of the current 

 increases from 8 miles to I if. From St. Augustine to the Bay 

 of New York the Gulf Stream takes a north-easterly course, 

 parallel with the land, and conterminous with a cold current 

 which flows from the north to the south between the stream and 

 the American coast. In this part of its course it continues to in- 

 crease in breadth from iif miles at St. Augustine to 3if at New 

 York ; meanwhile its speed decreases from 4 feet to 2I per second. 

 The depth of the sea along the course of the current is many 

 hundred fathoms, and the distance between St. Augustine and 

 New York is 180 miles. On quitting the Bay of New York, the 

 Gulf Stream takes an E.N.E. direction to the south of New- 

 foundland, skirting the cold current, which goes dowii to south- 

 west as far as New York, following the east coast of Newfound- 



land. By the time the Gulf Stream, after a course of 2CX3 miles, 

 reaches the south of Newfoundland, it has attained a breadth of 

 about 80 miles, while its speed is only 2 feet per second ; but the 

 current continues to run in the same direction towards Europe 

 for other 300 miles, with a speed which is from 2 feet to 06 

 feet, and a breadth increasing from 80 up to 200 miles. The 

 Gulf Stream, when it has attained a distance of 750 miles from 

 Bemini, separates into two branches, the one proceeding south- 

 wards towards the coast of Africa, at a speed of 06 feet per 

 second, the other taking a northerly course towards Iceland, 

 along the shores of the British Islands, and running about 200 

 miles, at a rate which decreases from 06 to 03 feet per second, 

 the breadth of the current meanwhile increa-.ing from 100 to 

 105 miles. When the stream reaches the neighbourhood of 

 Iceland, it sends off a branch which skirts the south coast 

 of that island, afterwards taking a direction north-west towards 

 the Polar current of the east coast of Greenland, which it seems 

 partly to follow in its march southwards. As to the main stream, 

 it inclines to the east after passing the extreme north of Scot- 

 land, and then runs to the north-east, along the west coast of 

 Norway, until it ends its wanderings in the Icy Sea. 



As to the Polar Current we feel authorised to mention the fol- 

 lowing statements : — From the region of the Icy Sea, the most 

 northerly of which we have any knowledge, from the neighbour- 

 hoo 1 of Spiizbergen about the 80th degiee of N. latitude, 

 there descends to the south-west a great polar current loadetl 

 with floating ice. It reaches the coast of Greenland at 

 about 70' N. latitude, and follows it as far as Cape Farewell ; 

 its breadth being nearly 40 miles and its speed f of a foot per 

 second. After passing Cape Farewell, it curves round to northward 

 and follows the west coast of Greenland for some distance into 

 Davis Strait. Alter having run for a few degrees in this direction it 

 bends to the south-west, towards the coast of Labrador, along 

 the whole length of which it runs, then proceeding to the 

 south-east, enlarged by the polar current which comes from 

 Baffin's Bay. On quitting Lalirador, where its speed is ^ of a foot 

 per second, and its breadth 50 miles, the polar current on round- 

 ing the east coast of Newfoundland makes for the Gulf Stream, 

 and, after doubling Cape Race, sends a branch to the south- 

 west between the Gulf Stream and the American coast, which 

 branch can be traced as far as Florida. As to the part of the 

 polar current which does not take this route, it is generally ad- 

 mitted that it flows underneath the Gulf Stream on the east of 

 Newfoundland, and that it runs uninterruptedly to the south-east, 

 towards the African coast, where the waters of the ocean are of 

 a temperature comparatively low. 



In order to explain the causes of these immense ocean currents 

 by the aid of the laws of the movement of water in ordinary 

 conduits, it is necessary first of all to know the forces 

 which produce and maintain the movement of these currents. 

 Captain Maury, who has made a special study of this question, 

 has given it as his opinion that these ocean currents are due to 

 the differences caused by the changes of temperature and of 

 saltness in the specific gravity of the water of the sea. In order 

 to make this theory more easy of comprehension, Maury imagines 

 a globe like the earth covered over the whole of its surface 

 with a sea 200 fathoms in depth, the water throughout being 

 of the same density ; at the same time he supposes the 

 surrounding circumstances to be the same at all points, 

 and that there being neither evaporation nor precipita- 

 tion, there can of course be neither winds nor currents upon the 

 imaginary globe. He next supposed the water contained between 

 the tropics suddenly transformed into oil to a depth of 100 

 fathoms. From this moment the equilibrium is destroyed, and 

 there results a general system of currents and counter-currents ; 

 for the oil, being lighter than the water, will rush along the 

 surface towards the poles, while the water of these regions makes 

 for the equator in the shape of a submarine current. As the oil 

 reaches the polar sea, it is supposed to be transformed into water, 

 which returns to the equator, where it is changed anew into oil 

 that again rises to the surface and again makes its way to the 

 poles, and so on. If then this globe turns, like the earth, on its 

 axis once in the twenty-four hours from west to ea^t, each par- 

 ticle of oil, according to Maury, will proceed towards the pole 

 in a spiral course with a speed towards the east always in- 

 creasing ; on reaching the pole it will turn at a rate equal ty 

 that at which the earth revolves at the equator, viz., 225 miles 

 an hour. But, says Maury, when the oil has been changed into 

 water, it will return towards the equator describing a curve in a 

 westerly direction. If the sea in question should be bounded by 

 land, as is the case on the surface of the earth, the uniformity of 



