CHAP. I.] THE EQUIPMENT OF THE SHIP. 83 
the latter case at intervals of 10, 50, or 100 fathoms, to determine the 
depth and volume of masses of moving water derived from different 
sources. 
The simple determination of the depth of the ocean at tolerably reg- 
ular distances throughout the entire voyage is an object of such primary 
importance that it should be carried out whenever possible, even when 
circumstances may not admit of dredging, or of any thing beyond 
sounding. ‘The investigation of various problems relating to the past 
history of the globe, its geography at different geological epochs, and 
the existing distribution of animals and plants, as well as the nature and 
causes of oceanic circulation, will be greatly aided by a more accurate 
knowledge of the contour of the sea-bed. 
Surface Temperature-—The surface-temperature of the sea, as also 
the temperature of the air as determined by the dry and wet bulb ther- 
mometers, should be regularly recorded every two hours during the day 
and night throughout the voyage. 
These records should be reduced to curves, for the purpose of ready 
comparison ; and the following points should be carefully attended to: 
1. In case of a general correspondence between the temperature of 
the sea and that of the air, it should be noted whether, in the diurnal 
variation of both, the sea appears to follow the air, or the air the sea. 
2. In case of a marked discordance, the condition or conditions of 
that discordance should be sought in (a) the direction and force of the 
wind, (4) the direction and rate of movement of the ocean surface-water, 
(c) the hygrometric state of the atmosphere. When the air is very dry, 
there is reason to believe that the temperature of the surface of the sea 
is reduced by excessive evaporation, and that it may be below that of 
the subsurface stratum a few fathoms deep. It will be desirable, there- 
fore, that every opportunity should be taken of comparing the tempera- 
ture at the surface with the temperature of the subsurface stratum, say 
at every five fathoms down to twenty fathoms. 
Temperature Soundings.—The determination of the temperature, not 
merely of the bottom of the ocean, over a wide geographical range, but 
of its various intermediate strata, is one of the most important objects 
of the expedition; and should, therefore, be systematically prosecuted 
on a method which should secure comparable results. The following 
suggestions, based on the experience already obtained, in the North At- 
lantic, are made for the sake of indicating the manner in which time 
and labor may be economized in making serial soundings, in case of the 
