802 
THE VOYAGE OF H.M.S. CHALLENGER. 
afterwards a gradual descent to 2600 fathoms, the mean depth being 2275 fathoms. 
From a position in lat. 40° 3' S., long. 133° W., towards the coast of South America 
the bottom is undulating, varying from 2600 to 1500 fathoms, the mean depth being 
2070 fathoms ; these soundings seem to indicate the existence of a ridge along the 
bed of the ocean, stretching away from the South American coast on or about the 
40th parallel of south latitude, but whether such a ridge does exist or not can only be 
ascertained by the slow process of deep-sea sounding. 
The bottom temperature on the section from Tahiti to the 40th parallel varied from 
34 °’7 to 3 5° '4, the mean being 35°'0 or nearly the same result as between Tahiti and 
Japan, but from the position in lat. 40° 3' S., long. 133° W., towards Mocha Island the 
range in the bottom temperature was 1°'2, notwithstanding which the mean result 
was 35° '0, or precisely the same as before. The results in the last section appear to 
confirm the existence of a ridge in this part of the Pacific, for it will be noticed that the 
temperatures obtained after the shoal cast of 1500 fathoms (see Diagram 20) agree 
remarkably well (35° - 4), whilst between the two shoal casts of 1600 and 1500 fathoms 
they fall to 34° '5, being again warmer west of the 1600 fathoms sounding, where the 
temperature was 35°'2, indicating probably that the ship was south of the ridge between 
the two casts of 1600 and 1500 fathoms, and north of it during the remainder of the 
section. 
In the section from Tahiti to the parallel of 40° S., the surface temperature changed 
gradually from 78° to 54° - 5, and in the section towards Mocha Island remained at a 
temperature of 54°’5 from the meridian of 133° to that of 101° W., after which it gradually 
rose to 58°, and was 58° at Juan Fernandez and 59° at Valparaiso. 
The serial temperatures show that in the section south from Tahiti, notwithstanding 
the change in the surface temperature from 78° to 54° - 5, the isotherm of 40° remains at 
about the same depth (550 fathoms), but slightly deeper in the south than in the north. 
Above the isotherm of 40° the isotherms gradually rise as the surface temperature 
becomes lower. In the section from a position in lat. 40° 3' S., long. 133° W., towards 
Mocha Island the isotherms are nearly parallel with the surface. The surface and 
consequently the upper temperatures are slightly higher in the eastern than in the 
western part of this section, but this difference is readily accounted for, not only by the 
advance of summer, but from the fact that polar winds with a northerly current were 
experienced in the western, and equatorial winds with a southerly current, in the eastern 
part. 
The currents on the passage were moderate, in no case exceeding 24 miles per day, 
the average rate being 11 miles per day, the direction of the current depending almost 
invariably on the direction of the wind. 
Anemometer observations were taken on any favourable opportunity when the ship 
was stationary, sounding or dredging. The following table shows the results recorded : — 
