132 
POPULAR SCIENCE REVIEW. 
of the Atlantic would be 55°, like that of the Mediterranean 
within the Strait of Gibraltar. But see what we get a little 
outside that basin. Near the coast of Spain, only 100 or 200 
miles from Gibraltar, we found the temperature 49° at 800 
fathoms, and we got down to 39° at 1,100 fathoms. Now this 
shows perfectly clearly that such a low temperature could only 
be sustained by a constant flow of water from the Polar basin 
towards this southern region. Then, as I have shown you, that 
outflow could not continue without an inflow into the Polar 
basin. And that brings me to show you what is the force that 
maintains this circulation. It is produced by the continual 
cooling of the water which flows into the Polar area ; for it 
becomes heavier and falls to the bottom, displacing the water 
previously there, pushing it away as it were. Thus, there is 
a constant sinking of water in the Polar area exposed to a much 
colder atmosphere ; for every fresh layer of water that comes 
in from the warmer sea around is cooled in its turn ; it then 
sinks and goes down, down, down ; and this colder and denser 
water creeps gradually along the deepest parts of the great At- 
lantic basin, and now and then, by some peculiar conformation 
of the bottom, it will come nearer to the surface, as it does in 
this Lightning Channel. If we are ever able to trace the 
Lightning Channel further north, it will be a most interesting 
point to determine what it is that sends up the cold water 
so much nearer the surface there than it has been found any- 
where else in the same latitude. But we have a parallel fact 
in the case of Gibraltar, where I have lately been able to prove 
very distinctly that the water from the deeper portion of the 
Mediterranean basin is passing as an under-current outwards 
through the shallowest part of the Strait, beneath the surface- 
current that is continually flowing inwards from the Atlantic. 
Thus, then, you see what is the moving force. It is this con- 
stant reduction of temperature, which increases the density of 
the water and disturbs the equilibrium too. Suppose we had a 
Polar column of water of a certain height at this end of the 
room, and an Equatorial column at the other end. As this 
Polar column is cooled down, it contracts and becomes denser ; 
thus its level is lowered, and the water will flow towards its 
surface to bring up that level. When this column of dense 
Polar water has on the top of it the additional water which has 
flowed in to maintain the level of that column, it becomes con- 
siderably heavier than the corresponding Equatorial column at 
the other end. What is the consequence ? Why, that a portion 
of the lower part of it must flow away. Thus there will be a 
tendency to a renewed lowering of the level, which must draw 
in water from the Equatorial region ; and there will always 
be, as that water flows in and is cooled down, a tendency to 
