628 
MR. J. PRESTWICH ON SUBMARINE TEMPERATURES. 
Lenz then proceeds to observe : — “ The form of the submarine isothermal line which 
I have drawn leads us of itself, on the first glance, to an explanation of this striking 
phenomenon. 
“ The mass of water in the tropics, warmer down to a certain depth from the sun’s 
heat, cannot maintain its equilibrium with the colder waters of the middle and higher 
latitudes : a flow of the warmer water from the equator to the poles must necessarily 
take place on the surface ; and this surface-flow must be supplied at the equator by a 
flow of colder water from high latitudes, which at first would flow in an almost hori- 
zontal direction, but which under the equator must rise from below to the surface. In 
this manner, in the northern hemisphere, a great vertical circulation takes place in the 
ocean, which has its direction above from the equator to the pole, and below from the 
pole to the equator. Since these flows or currents moving in opposite directions are 
distinguished by their different temperatures, we obtain in the submarine isotherm an 
indication of the direction of the lower portion of this flow. A corresponding flow, but 
moving in the opposite direction, takes place in the southern hemisphere ; so that in a 
zone surrounding the equator where both are united, the water flows almost in the 
direction from below up to the surface ; and thus one meets with cold water in much 
shallower depths than in those two zones north and south which lie immediately 
adjoining, and which, in fact, is shown by the observations. 
“ It is not my intention to enter here upon the question, how the original direction 
of this current to the surface becomes greatly altered by the diminution of the speed of 
rotation and by the influence of the wind, so that the water first arrived at the polar 
regions by considerably circuitous ways, or how the lower portion of the current was 
drawn westward by the entrance of bodies of water into latitudes of greater speed of 
rotation ; in any case the last influence will be very much diminished by the opposition 
of the west bank of the ocean, in comparison with the corresponding diversion or 
drawing away which the air-currents undergo. It is sufficient for me to have furnished 
in the figure of the submarine isothermal line proof of the current from the pole to the 
equator in the depth of the ocean. It would be highly desirable that future navigators 
should enlarge our knowledge on this point, by a larger number of observations with 
one and the same instrument, or with corrected instruments, which could be accom- 
plished with very little trouble and in a very short time. If they would be satisfied 
with letting down the thermometrographs at always one and the same depth of some 
100 fathoms, this observation would be made in fifteen minutes; and in any case, by a 
frequent repetition of it, results would be arrived at, especially in latitudes ranging 
from 40° N. to 40° S., which would be far more instructive for physical geography than 
the observations hitherto made, where one proceeded or reasoned more on the deter- 
mination of the diminution of the temperature than upon compared determinations of 
different places. 
“ From a current underneath of colder waters from the poles to the equator, some 
important conclusions arise, viz. : — 
