TEMPERATURE OF OCEAN. 



of invariable temperature, the depth of which at the highest 

 latitudes at which observations have been made, is estimated at 

 about 4500 feet. 



35. It might be imagined that the temperature of the surface 

 would be propagated downwards, and that a thermal equalisation 

 might therefore be produced by the intermixture of the superior with 

 the inferior strata, arising from the agitation of the surface of the 

 waters by atmospheric commotions. It is found, however, that 

 these effects, even in the case of the most violent storms and 

 hurricanes, extend to no great depth, and that while the surface 

 of the ocean is furrowed by waves of the greatest height and 

 extent, the inferior strata are in the most absolute repose. 



36. If water followed the general law, in virtue of which all 

 bodies become more dense as their temperature is lowered, a con- 

 tinued frost might congeal the ocean from its surface to the 

 bottom, and certainly would do so in the polar regions ; for in that 

 case the system of vertical currents, passing upwards and down- 

 wards, and producing an equalisation of temperature, which has 

 been shown to prevail above 38*8, would equally prevail below 

 that point, and consequently the same equalisation of temperature 

 would be continued, until the entire mass of water, from the 

 surface to the bottom, would be reduced to the point of congela- 

 tion, and would consequently be converted into a solid mass, all 

 the organised tribes inhabiting the waters being destroyed. 



The existence of a temperature of maximum density at a point 

 of the thermometric scale above the point of congelation of water, 

 combined with the very feeble conducting power of water, 

 whether in the liquid or solid state, renders such a catastrophe 

 impossible. 



37. The air is subject to less extreme changes of temperature at 

 sea than on land. Thus, in the torrid zone, while the temperature 

 on land suffers a diurnal variation amounting to 10, the extreme 

 diurnal variation at sea does not exceed 3. In the temperate 

 zone the diurnal variation at sea is limited generally to about 5|, 

 while on continents it is very various and everywhere consider- 

 able. In different parts of Europe it varies from 20 to 25. 



At sea as on land the time of lowest temperature is that of 

 sunrise, but the time of greatest heat is about noon, while on 

 land it is at two or three hours after noon. 



On comparing the temperature of the air at sea with the super- 

 ficial temperature of the water, it has been found that between the 

 tropics the air, when at its highest temperature, is warmer than 

 the water, but that its mean diurnal temperature is lower than 

 that of the water. 



In latitudes between 25* and 50 the temperature of the air is 



77 



