ME. J. PEESTWICH ON STTBMAEINE TEMPEEATUEES. 
623 
§ IV. Hypotheses of Humboldt, Arago, Lenz, and others. 
Such is a summary of the results obtained between the years 1749 and 1868. From 
time to time they had been commented on by some of the most eminent physicists of 
the time, and the cause of the low temperatures prevailing in the depths of tropical seas 
discussed. 
Humboldt, so far back as 1812, and again in his subsequent works* * * § , contended that 
“the existence of those cold layers in low latitudes proves the existence of an under- 
current flowing from the poles to the equator.” In support of this hypothesis, he showed 
how it explained the fact, first noticed by Franklin and Williams f, that the water on 
shoals in the Atlantic was many degrees lower than that surrounding them, from the cir- 
cumstance that the deeper cold water, flowing and rising over them, displaced the warmer 
surface-waters J. These observations were afterwards confirmed by Hu Petit-Thouars, 
Vaillant, and others. He was further of opinion that “ in the narrower seas, as well 
as in the tropical seas which cover the cold waters from Arctic regions, all the mass of 
water is in a state of movement.” 
Humboldt also contested the conclusions of those who considered that the ocean is 
salter under the equator than at a distance from it, and showed that while in lat. 0° to 
14° the specific gravity was T0272, it was 1-0282 in lat. 15° to 18°, and 1-0278 in lat. 
30° to 40°. Nor did he fail to note § that the equatorial zone is not the hottest water 
zone ; but that two hotter zones lie a few degrees N. and S. of it. 
Humboldt subsequently [| thus summarized the question as it then stood : — 
“ As fresh and salt water do not attain the maximum of their density at the same 
degree of temperature, and as the saltness of the sea lowers the thermometrical degree 
corresponding to this point, we can understand how the water drawn from great depths 
of the sea during the voyages of Kotzebue and Du Petit-Thouars could have been 
found to have only the temperature of 37° and 36°-5. This icy temperature of sea-water, 
which is likewise manifested at the depths of tropical seas, first led to a study of the 
lower polar currents, which move from both poles towards the equator. Without these 
submarine currents the tropical seas at those depths could only have a temperature 
equal to the local maximum of cold possessed by the falling particles of water at the 
radiating and cooled surface of the tropical sea. In the Mediterranean the cause of the 
absence of such a refrigeration of the lower strata is ingeniously explained by Arago, 
* ‘ Voyage 5 : Eelation Historique (Paris 1814), vol. i. p. 73. Climatologie Asiatique (Paris 1831), p. 560. 
1 Kosmos,’ Otto’s translation, 1849, vol. i. p. 307. 
f On the Use of the Thermometer in Navigation. Philadelphia, 1792. 
J He instances, for example, a case noticed by himself on the “ Signal Bank” off Eerroll, where he found 
the water to have a temperature of from 54°-5 to 56° F., while the water immediately around was from 59° to 
59°-6 F. 
§ Ann. Chim. et Phys. xxxiii. 1820, p. 40. 
I] Kosmos, vol. i. pp. 308, 309 (Sabine’s translation, pp. 295, 296). 
