ME. J. PEESTWICH ON SUBMAEINE TEMPEEATUEES. 
615 
Wilkes, who also explored the Antarctic seas in 1838—42, took the same view, and 
for the same reasons as D’Urville, Wauchope, and Eoss, of the existence of a deep-sea 
and of a belt of water of the uniform temperature of 39 0- 5 F. 
Commenting on the general results of this great American expedition, Biot discusses* 
the question of deep-sea temperatures. He remarks that serial observations should in 
all cases be made, “ that the instruments ought to be protected against pressure by 
surroundings of great strength and resistance,” and that they should be left a consi- 
derable time at the bottom. Comparing the observations of Eoss with those of 
Du Petit-Thouars, Scoresby, Parry, and Martins, he shows their want of agreement. 
He says that the experiments of Eoss depend entirely on his instruments, “ of which 
he had no means of knowing and judging ( aucun moyen d' appreciez)” while he knew 
those of Du Petit-Thouars and Martins to have been prepared with every care. For 
Eoss, he remarks, “ the uniformity of temperature at the bottom of the sea is a necessity; ” 
and he trusts that some steps may be taken to verify his observations, for between them 
and those of other observers there is, he remarks, “ a complete incompatibility.” 
With respect to the freezing-point and point of greatest density of sea-water, these 
properties were first more particularly investigated by Dr. Marcet in his well-known 
paper on the subject published inl819f. Dr. Marcet ascertained that he could lower 
the temperature of sea-water (at 1-027 sp. gr.) to 27°, and even, when in large vessels and 
kept perfectly still, to 18° or 19° F., before freezing, but that when it froze it always rose 
to 28°; and he states that his experiments “ uniformly led him to the conclusion that 
the law of greatest specific density at 40° did not apply to sea-water, but that, on the 
where the mean temperature of the sea obtains throughout its entire depth, forming a boundary, or kind of 
neutral ground, between the two great thermic basins of the oceans. To the north of this circle the sea has 
become warmer than its mean temperature, by reason of the sun’s heat which it has absorbed, elevating its tem- 
perature at various depths in different latitudes. So that the line of mean temperature of 39 0, 5 in latitude 
45° S. has descended to the depth of 600 fathoms ; and at the equatorial and tropical regions this mark of the 
limit of the sun’s influence is found at the depth of about 1200 fathoms, beneath which the ocean maintains its 
unvarying mean temperature of 39°-5, whilst that of the surface is about 78°. 
“ So likewise, to the south of the circle of mean temperature, we find that, in the absence of an equal solar 
supply, the radiation of the heat of the ocean into space occasions the sea to be of a colder temperature as we 
advance to the south ; and near the 70th degree of latitude we find the line of mean temperature has descended 
to the depth of 750 fathoms, beneath which again, to the greatest depths, the temperature of 39°-5 obtains, 
whilst that of the surface is 30°. 
“ The experiments which our limited time and means admitted of our making serve to show that the mean 
temperature of the ocean at present is about 39°*5, or 7| degrees above the freezing-point of pure water, and 
as nearly as possible the point of its greatest density. But it would be indispensable that this temperature 
should be ascertained to the tenth part of a degree ; and as we now know where we may send any number of 
thermometers down to the greatest fathomable depths without an alteration of temperature, even to that small 
amount, this desideratum might be very easily obtained.” (Boss’s ‘ Yoyage to the Antarctic Kegions,’ vol. ii. 
p. 375.) 
* Journal des Savans, 1849, p. 69. 
t “ On the Specific Gravity and Temperature of Sea-water,” Phil. Trans, for 1819, p. 161. 
