194 THE GULF STREAM. 
The temperature of the oceanic 
a 
£ 
g eae etre he basin depends upon the depth, the 
= ° . 
Tg er ae ae a aa latitude, the currents, and the sea- 
390° 
sons; that of mediterraneans (land- 
locked seas) is controlled by other 
causes, Which will be more fully dis- 
cussed when we come to treat of the 
temperature of the Caribbean and 
of the Gulf of Mexico. The constants 
are the depths and latitude, whilethe 
disturbing elements are represented 
by the varying atmospheric and oce- 
: anic currents and the seasons.* The 
effects of seasonal differences of tem- 
perature do not extend to great 
depths, yet act with sufficient power 
greatly to modify the force and vol- 
ume of the oceanic currents. AS a 
general rule, the temperature dimin- 
ishes from the surface toward the 
60° 70° 80° 
50° 
20° 
1o° 
nee 
i 5 bottom, a belt limited in depth (about 
& ™ 150 fathoms) alone, being subject to 
2 variations due to the action of the 
sun. Below that the temperature 
2 generally decreases with the depth, 
until we reach the body of water of 
9 which the temperature may in gen- 
eral be said to be uniform (about 
9 3D°),T 
As explanations of the oceanic cur- 
3 rents, we have first the gravitation 
theory, which looks upon the difter- 
3 ences of temperature and of specific 
gravity of the water at the equator 
and poles as the prime cause of oce- 
anic circulation; next, Thomson’s 
theory, according to which the differ- 
ence in evaporation and precipitation 
between the northern and southern 
Boas. 
7° 
80° 
90° 
y Ba 2 8 5 ; 
3 Bete a) oo ee hemispheres causes a consequent 
= 4 . ° 
E heaping up of water in the south- 
* Dr. J.J. Wild has given in “ Thalassa” an excellent diagram, showing ata glance 
the general relations of the temperature in the liquid envelopes to the earth’s crust. 
It is here re-produced (Fig. 2), slightly modified. 
+ As currents sink,as soon as their temperature falls below that of adjoining waters, 
and as the temperature diminishes from the surface toward the bottom, as well as 
from the equator to the pole, a combination of these varying elements may produce 
a somewhat complicated circulation, 
pie ol sr et 
Sy ae Se 
