1872.] Notices of Books. 355 
can be explained by the action of difference in specific gravity 
alone; the difference in temperature causes a difference in 
specific gravity, and for the restoration of the equilibrium a 
surface current of the lighter flows towards the denser water, 
whilst at the same time an under-current of the latter flows 
towards the former. The expansion caused by heat tends to 
raise the level at one end, and the contraction caused by cold 
tends to depress it at the other end; but if this difference of 
level were produced without any difference in the specific gravity, 
it would, in an open ocean, as already shown, be restored not by 
currents, but by a tidal movement of the ocean, the higher level 
sinking and the lower rising simultaneously. 
‘Tt appears that in the Mediterranean the surface heat, resulting 
from the action of the sun’s rays, causes evaporation, which 
lowers the level and increases the density of water; and therefore 
when, in treating of the general oceanic circulation, Dr. Carpenter 
says that the level of Polar water is reduced and its density 
increased by surface cold, it might be expected that he would 
admit that the same effects are caused in the Equatorial regions 
by surface heat, or at least give some explanation as to why it 
should be otherwise. But instead of this Dr. Carpenter, with- 
out explanation, deliberately asserts that in the Equatorial regions 
the level of the water is raised, and its density diminished by the 
surface heat to which it is exposed: that is to say, that surface 
heat in the Equatorial regions causes exactly the opposite effects 
to those which it causes in the Mediterranean. If the observed 
effects in these two regions are really so diametrically opposite, 
can they be the result of the same cause? or may not the 
difference in the effects rather be the result of a difference in 
the ratio which the evaporating action of the sun’s rays in the 
two places bears to the respective amounts of fresh water 
supplied by rivers and rain? But has it ever been shown that, 
whatever be the cause, there actually are such different effects 
in the two regions, or is this another reckless assumption to suit 
the exigencies of a preconceived theory ?”’ 
But by no means do Dr. Carpenter’s anomalous deductions end 
here. Besides the vertical circulation resulting from differences 
in temperature, Dr. Carpenter says that ‘‘the Gulf Stream forms 
part of a horizontal or superficial circulation in the North 
Atlantic, of which the Trade Wind constitutes the primum mobile.” 
Thus the general features of the theory seem to be that the 
difference of temperature in Polar and Equatorial regions 
causes a vertical, and the Trade Winds a horizontal circulation 
throughout the ocean. This is brought forward not merely as 
a theory by which a system of circulation might be caused, but 
as an efficient cause of the circulation which actually exists in 
the ocean. 
Three years ago Mr. Jordan, making use of his own and Captain 
Maury’s arguments, endeavoured to show the inefficiency of the 
