646 CIECULATION OF THE OCEAN. 



for doing so is continually increasing. Hence the great concentration of 

 the water in the steady Trades of the Atlantic. 



The observations make it probable that in the Atlantic the water from 

 the surface, down to a depth of 1,000 fathoms, has, on the whole, a flow 

 inwards, or from South to North, and below that depth and down to the 

 bottom it appears to have an opposite flow, thus providing for the removal 

 of the salt which otherwise would accumulate in the North Atlantic. The 

 Atlantic thus presents, on a larger scale, what is observed in the Mediter- 

 ranean, where the mean drying power of the atmosphere is higher than 

 even in the North Atlantic. In the Pacific, owing to its form and general 

 climate, these conditions are not so evident. 



The Gulf Stream (or rather the edge of it, where its warm and dense 

 water met the " cold wall " of the Labrador Current), was very clearly 

 indicated simply by the Specific Gravity. The Equatorial Currents, also, 

 were very marked. The Agulhas Current of South Africa showed similar 

 variations in density. 



CIRCULATION of the OCEAN. — That there is a circulation and inter- 

 mingling of all the Ocean waters is shown in (246), page 297. Of the 

 means by which this interchange is produced, numerous theories have been 

 advanced, as alluded to in (248), page 298, and in (362) and (363), pages 

 377 — 379. It is only in recent years that any reaUy reliable data have 

 existed to elucidate the subject, and, at the present time, physicists are 

 pretty well agreed that the great cause of the circulation of the water is 

 its varying Specific Gravity. This Specific Gravity, or Density, is regu- 

 lated chiefly by Temperature, but also by the amount of salt contained in 

 the water. "Without going into further details, the simple theory, which 

 recent investigations have strengthened, is that a cold under-flow of water 

 is continually kept up from the Polar to the Equatorial regions.* 



This theory Professor Lenz, of St. Petersburg, advanced as an inevitable 

 deduction from the facts ascertained by the remarkable series of observa- 

 tions on the Temperature and Specific Gravity of Oceanic Water at various 

 depths, which he had made in the second voyage of Kotzebue, during the 

 years 1823 — 1826. He drew from these results the very conclusions which 

 derived remarkable confirmation from the Temperature soundings and 

 Specific Gravity observations of H.M.S. Challenger, viz. : — 1. The doctrine 

 of a deep under-flow of glacial water from each Pole to the Equator. 

 2. The ascent of Polar water towards the surface under the Equator, as 



• That under -currents do actually exist is well-known (see pages 407 — 409). H.M.S. 

 Challenger several times tested the rate and direction of the under-current by means of 

 a wooden framework, covered with canvas, lowered by a rope to the depth at which the 

 current was to be tested ; the upper end of the rope was attached to a buoy wh'oh 

 floated on the surface, and, after allowing for the efiect of the surface-current, showed 

 by its progress the direction of the current at the depth where the framework was 

 floating. This same process was used by Dr. Carpenter, in testing the flow of cold 

 water from the Atlantic into the Mediterranean through the Straits of Gibraltar. In 

 recent years, the telegraph steamers have found evidences of strong currents at a depth 

 of 1,000 fathoms between the Canary Islands. It is, of course, possible that under- 

 currents may in places be uo near ibe burfuce as to aflect the oouise of a ship deep iji 

 the water. 



