t68 M. Humboldt m isothermal Lines ^ 
perature of the water at the surface corresponding to different 
latitudes, the ocean being considered at rest, and destitute of 
shallows and currents, The decrease of heat in the super- 
imposed strata of water. The effect of billows on the tem- 
perature of the surface water. The temperature of cur- 
rents, which impell with an acquired velocity, the waters of our 
zone across the immoveable waters of another zone. The re- 
gion of warmest waters no more coincides with the Equator, than 
the region in which the waters reach their maximum of saltness. 
In passing from one hemisphere to another, we find the warmest 
watei::s between 5® 45' of N. Lat., and 6° 15' of S. Lat. Per- 
rins found their temperature to be 82® .8 ; Quevedo 83®, 5 ; 
Churruca 83°.75 and Rodman 83°. 8. I have found them in 
the South Sea to the east of the Galapagos Isles 84®. 7. The va- 
riations and the mean result do not extend beyond 1°.3. It 
is very remarkable that in the parallel of warmest waters, the 
temperature of the surface of the sea is from 3°.6 to 5®. 4 
higher than that of the superincumbent air. Does this difference 
arise from the motion of the cooled particles towards the bot- 
tom, or the absorption of light, which is not sufficiently compen- 
sated by the free emission of the radiant coloric. As we ad- 
vance from the Equator to the Torrid Zone, the influence of 
the seasons on the temperature of the surface of the sea be- 
comes very sensible ; but as a great mass of water follows very 
slowly the changes in the temperature of the air, the means of 
the months do not correspond at the same epochs' in the ocean 
and in the air. Besides, the extent of the variations is less in 
the water than in the atmosphere, because the increase or de- 
crease in the heat of the sea takes place in a medium of varia- 
ble temperature, so that the minimum and the maximum of the 
heat which the water reaches, are modified by the atmospheri- 
cal temperature of the months which follow the coldest of the 
warmest months of the year. It is from an analogous cause, 
that in springs which have a variable temperature, for example, 
near Upsal^, the extent of the variations of temperature is only 
19®. 8, while the same extent in the air from the month of Janu- 
ary to August, is 39®. 6 . In the parallel of the Canary Islands, 
• Gilbert’s Annalen^ 1812, p. 129. 
