XATUJ7AL PHILOSOPHY. 157 



the land. Secondly, we have to consider that the whole bottom of 

 the sea is covered by a layer of ice-cold water, which moves constant- 

 ly from the poles to the equator, and which, in its passage over sand- 

 banks, causes, as Humboldt aptly remarks, the low temperatures 

 which are generally observed in shallow places. That the water near 

 the bottom ol'thc sea, on account ol'its great specific heat and its low 

 temperature, is better litted than the atmosphere to withdraw the heat 

 from the earth, is a point which requires no further discussion. 



We have plenty of observations which prove that the earth suffers a 

 great loss of heat through the waters of the ocean. Many investiga- 

 tions have demonstrated the existence of a large expanse of sea, much 

 visited by whalers, situated between Iceland, Greenland, Norway, 

 and Spitsbergen, and extending from latitude 70 to 80 N., and from 

 longitude 15 E. to 15 \V. of Greenwich, where the temperature was 

 observed to be higher in the deeper water than near the suriaee, an 

 experience which neither accords with the general rule, nor agrees 

 with the laws of hydrostatics. Franklin observed, in latitude 77 N., 

 and longitude 12 E., that the temperature of the sea near the surface 

 was , and at a depth of 700 fathoms-j-G u . Fisher, in latitude 80' J 

 N. and longitude 11 E., noticed that the suriace-water had a tem- 

 perature of 0, whilst at a depth of 140 fathoms it stood at -{-8. As 

 sea-water, unlike pure water, does not possess a point of greatest 

 density at some distance above the freezing-point, and as the water 

 in latitude 80 Isl. is found at some depth to be warmer than water at 

 the same depth 10 southward, we can only explain this remarkable 

 phenomenon of an increase of temperature with an increase of depth 

 by the existence of a source of heat at the bottom of the sea. The 

 heat, however, which is required to warm the water at the bottom of 

 an expanse of ocean more than 1,000 square miles in extent to a sensi- 

 ble degree, must amount, according to the lowest estimate, to some 

 cubic miles of heat a day. The same phenomenon has been observed 

 in other parts of the world, such as. the west coast of Australia, the 

 Adriatic, the Lago Maggiore, &c. Especial mention should here be 

 made of an observation by Horner, according to whom, the lead, 

 when hauled up from a depth varying from 80 to 100 fathoms in the 

 mighty Gulf-stream off the coast of America, used to be hotter than 

 boiling water. 



The facts above mentioned, and some others which might be added, 

 clearly show that the loss of heat suffered by our globe during the last 

 2,600 years is far too great to have been without sensible effect on the 

 velocity of the earth's rotation. The reason why, in spite of this ac- 

 celerating cause, the length of a day has nevertheless remained con- 

 stant since the most ancient times, must be attributed to an opposite 

 retarding action. This consists in the attraction of the sun and moon 

 on the liquid parts of the earth's suriaee. 



According to calculation the retarding pressure of the tides against 

 the earth's rotation would cause, during the lapse of 2,500 years a 

 sidereal day to be lengthened to the extent of y^- of a second ; as the 

 length of a day, however, has remained constant, the cooling effect 

 of the earth during the same period of time must have shortened the 

 day iV of a second. A diminution of the earth's radius to the 

 amount of 4 meters in 2,500 years, and a daily loss of 200 cubic miles 



