REPORT ON OCEANIC CIRCULATION. 
19 
cause operates strongest during those months when, as in the case of Australia, 
atmospheric pressure is lower in the interior than on the coast. Those winds, of 
course, blow the warm waters toward the coast. The other cause is the back water, or 
counter equatorial current to the east, which sets in particularly in the eastern division of 
the region of calms between the north and south trades. 
In the Pacific, in equatorial regions, a great breadth of that ocean from Galapagos 
Islands to long. 165° E. has a temperature under the mean, falling to 50° in the middle 
region from long. 122° to 148° W. This low temperature of 50°, taken in connection 
with the isothermal of 72°, only eighteen degrees of latitude to the south, presents 
what may be regarded as perhaps the most striking fact of oceanic circulation. In 
August 1875 the Challenger took several soundings in this part of the ocean, and in 
Number 6 of the Preliminary Reports there is represented on Plate IV. the vertical 
distribution of temperature in this region as contrasted with surrounding regions. 
The following extracts from the Challenger soundings. Nos. 410 and 417, taken 
respectively on August 30 and September 14, 1875, will show the extraordinary 
difierences in the rates of change of temperature with depth in these two parts of 
the ocean down to 200 fathoms ; — 
Depth in 
Fathoms. 
No. 410. 
Lat. 7“ 35' N. 
Long. 149° 49' W. 
No. 417 
Lat. 11° 20' S. 
Long. 150° 30' W. 
Difi'erence. 
Surface. 
81°-0 
80° -0 
+ r-o 
10 
79° -3 
79°-9 
-0°-6 
20 
79° -3 
79°-8 
-0°-5 
30 
79°-3 
79°-6 
-0°-3 
45 
78°-2 
79°-4 
- 1°‘2 
50 
74°-0 
79° -0 
-5°-0 
60 
63°-4 
78°-0 
-14°-6 
70 
56°-2 
76°-7 
- 20°-5 
80 
52°-6 
75° -3 
-22°-7 
90 
51°-0 
73° -7 
-22°-7 
100 
50°-3 
72°-0 
-21°-7 
no 
50° -0 
70*-l 
-20°-l 
120 
49°-8 
68°-3 
-18"-5 
130 
49° -6 
66°-2 
-16°-6 
140 
49°-4 
64°-0 
-14°-6 
150 
49°-2 
61°-4 
- 12‘’-2 
160 
49°-0 
58°-9 
-9°-9 
170 
48°-8 
56"-4 
-7°-6 
180 
48°-6 
54°-2 
-5°-6 
190 
48°-4 
52°-4 
-4°-0 
200 
48°-2 
50° -8 
-2*-6 i 
