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POPULAR SCIENCE REVIEW. 
the bottom (640 fathoms) was a river, so to speak, of glacial 
water nearly 2,000 feet deep — below the freezing point of fresh 
water. Now that was the very curious fact which our investi- 
gations of this channel between the Faroe Islands and Orkney 
and Shetland brought to our knowledge. That channel I 
have been accustomed to designate the u Lightning Channel,” 
u Lightning ” being the name of the vessel assigned to us in our 
first expedition. This cold stream must have come straight into 
this channel from the Polar area ; but over it there was another 
stream proceeding north-east, consisting of water warmer than 
the normal water of the latitude ; for this last would have been 
about 40°, while the temperature of the surface was about 52°, 
and even at 500 fathoms it only sank to 45°. 
Now, then, what is the meaning of this ? When I speak of a 
u stream ” and “ flowing,” you must understand that there is 
nothing like a visible movement. I say that this cold stream must 
be flowing, because if it were not flowing it could not retain its 
temperature ; it would soon give up its warmth to the water 
above. It is quite a physical necessity that it should be in 
movement ; and of course if it is in motion, only by coming 
f ;om the Polar area could it have brought this cold temperature 
with it, for at the bottom it was about 29^ degrees. You are 
aware that 32 degrees is the freezing point of fresh water ; but 
it is not the freezing point of salt water. Sea water freezes at 
about 27° ; if it is kept very still it will not freeze till 25° ; and 
there is a most important difference in the condition of sea 
water and fresh water as regards temperature below 39*2 degrees. 
You all know perfectly well that when a frost acts upon the 
surface of a lake, river, or pond, the water freezes on the 
surface ; and if you put down a thermometer into the water 
below, you will find that its temperature is about 39 degrees. 
Now, why is this ? You know that the ordinary rule of the 
contraction of water is that it shrinks, just like the mercury in 
a thermometer, with cold, and expands with heat. As it 
shrinks it becomes denser, and therefore heavier, bulk for bulk ; 
consequently when a low atmospheric temperature is acting 
upon the surface of a pond or lake, the water as it is cooled at 
the surface becomes heavier and goes down. So it keeps on 
going down, while the warmer water beneath, which is lighter, 
comes up to the surface, till the whole is cooled down to about 
39*2 degrees ; but then continued cold does not produce the 
same effect, for below 39*2 the water begins to expand again, 
the greater cold making it lighter instead of heavier ; conse- 
quently the water which is cooled to below 39*2 degrees remains 
on the surface, and by continued exposure to the action of the 
cold atmosphere it freezes and forms a layer of ice. But that 
is not the case with salt water. Sea water continues to contract 
