beneath the surface of the Earth. 15 
from the surface, sources of heat amply sufficient to raise the air and 
water to the temperature they are found to possess at great depths. 
These objections have been severally and satisfactorily answered, and 
the last and most ingenious: of them has beer ably refuted by Mr. 
Fox,* who has shown, that in the Cornish mines the ascending has 
generally a Aigher temperature than the descending currents. The 
difference varied from 9° to 17°, showing that, instead of imparting 
heat, the currents of air actually carry off a large quantity of heat 
from the interior of the mines. 
It has been objected also to the doctrine of a central fire, that we 
perceive no traces of increase of temperature in the ocean at great 
depths, Now, in the consideration of this point, two principles are 
involved, first, that to which Mr. Fox has adverted, that the strata at 
the bottom of the ocean, were they composed of solid rock the most 
favorable for transmitting heat, would yet propagate it much more 
slowly than the water which covers it, and thus all accumulation must 
necessarily be prevented; second, the principle of the maximum 
density of water. 
All the observations hitherto made on masses of fresh water show, 
that at great depths} the temperature differs only a very few degrees 
from the point of maximum density. At greater depths it will prob- 
ably be found to be very near. that point. The latest experiments 
make the maximum density of pure water at about 38.75°, while those 
of Hallstrom make it, 39.38°. If the water be impure the point of 
maximum density will fall more or less, according to the nature and 
amount of the foreign bodies it may hold in solution. The experi- 
ments of Exmann show that the point of greatest density sinks very 
rapidly as we add any saline ingredient. 
Now, as the heaviest parts of any fluid will always find their way 
to the bottom, the deeper we de in a mass of water, either salt 
or fresh, we must find it the colder until we come to the limit of 
greatest density. In fresh water lakes of great depth the tempera- 
ture of the water will decrease as we descend till we reach the limit 
of 39° Fah. when the temperature will undergo no farther change. 
In salt water the point of maximum density increases with its saltness 
towards the poles,} so that the depth at which the temperature be- 
* Phil. Mag. and An. Feb. 1830 
¢ At 1000 feet Saussure found the lake of Geneva to have a temperature of 42° F. 
t See this Number, page 11. ; pens eo 
