274 



SCIENCE. 



[N. S. Vol. III. No. 6U. 



densation of the warm surface water, which 

 sinks into greater depths and imparts a 

 higher degree of temperature and salinity 

 to the substrata than are met with in any- 

 other ocean. The waters of these substrata 

 having a temperature from 60 to 64 degrees, 

 at a depth of 250 fathoms, meet the cold 

 waters, in a space about 40 miles wide, de- 

 scending along the edge of the continental 

 slope, which at the same depth (250 

 fathoms) have a temperature of only about 

 45 degrees. 



Within this space of forty miles' width a 

 transition of heat and salt is eiiected, result- 

 ing in an entire reconstruction of the super- 

 incumbent stratum of water, producing that 

 peculiar distribution of salt and heat at the 

 surface that is characteristic of the Gulf 

 stream. When warm seawater comes, into 

 contact with colder seawater it becomes 

 heavier, for the reason that the increase of 

 density, due to loss of heat, surpasses the 

 decrease, due to the loss of salt. When this 

 occurs in the depths of tlie ocean the warm 

 water will sink to still greater depths, but 

 here (as also on the slopes of great sub- 

 marine banks like the Bahama, Florida and 

 Campeche Banks) this dense and warm 

 water touches bottom, and another shift 

 must be made to dispose of the excess of 

 salt, the maintenance of equilibrium being 

 a physical necessity. 



The density of warm water is less affected 

 by the addition of a certain quantity of 

 salt than cold water would be, and for this 

 reason the excess of salt and heat at the 

 bottom, on the inner edge of the Gulf 

 Stream, shifts to higher levels where, in 

 consequence of higher temperatures, larger 

 quantities of salt can be stowed away with 

 less change of density than at greater depths. 

 Thus, by a withdrawal of salt and heat 

 from the greater depths and their accu- 

 mulation at the surface, that peculiar distri- 

 bution is attained which characterizes all 

 the serial temperature observations of the 



Gulf Stream sections, including those ob- 

 tained by the Challenger. 



Observations show the highest specific 

 gravities of the Gulf Stream waters to be in 

 the latitudes of Capes Lookout and Hat- 

 teras, exceeding those of all other parts of 

 the open ocean, and surpassed only by those 

 of the Red Sea and of the western part of 

 the Mediterranean. 



Although the ' upheaval ' of the waters of 

 the Gulf Stream develops first in upward 

 currents, in the substratum in which the 

 transition of heat and salt begins, it is not 

 improbable that these currents, like the 

 winds in aerial circulation, may assume a 

 more or less horizontal direction in their 

 progress to the surface. It may also be as- 

 assumed that the storage of heat in the sur- 

 face stratum is not without influence upon 

 the level of the Gulf Stream, and that this 

 difference of level between the Gulf Stream 

 and the adjacent areas of the ocean may call 

 other currents into life, but a farther consi- 

 deration of these subjects would lead us 

 into the sphere of the so-called dynamics 

 of the Gulf Stream, a field already ably 

 discussed and sufliciently studied. 



A. LiNDENKOHL. 



AN OPTICAL ILLUSION. 

 The brilliant electric lights on the borders 

 of the lake in the Baltimore park have served 

 to call my attention to a phenomenon which 

 is so very familiar that one is wholly disin- 

 clined to regard it as a 'phenomenon' at aU. I 

 refer to the fact that the long stream of light 

 reflected by the surface of the water from a 

 lamp on the opposite side does not look like 

 an object Ij'ing upon the surface, but like a 

 bright post projecting down into the water, 

 in continuation of the lamp-post. This is 

 without doubt a particular case of the illu- 

 sion by which lines which have any position 

 whatever in planes passing through the axis 

 of the body (or, for small near objects, in 

 planes passing through the vertical meridian 



