24 THE VOYAGE OF H.M.S. CHALLENGER. 



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 ocean-bed. 



Surface-Temperature. — The surface-temperature of the sea, as also the temperature of the air as determined 

 by the dry- and wet- bulb thermometers, 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, (b) the direction and rate of movement of the ocean surface-water, 

 (c) the hygrometric state of the atmosphere. When the air is very dry, tliere is reason to believe that the tempera- 

 ture 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, therefore, that every opportunity should be taken 

 of comparing the temperature at the surface with the temperature of the subsurface stratum, — say at every 

 5 fathoms down to 20 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 compar- 

 able results. The following suggestions, based on the experience already obtained in the North Atlantic, are 

 made for the sake of indicating the manner in which time and labour may be economised in making serial 

 soundings, in case of the employment of the Miller-Casella thermometer. They will be specially applicable to 

 the area in which the work of the Expedition will commence ; but the thermal conditions of other areas may 

 prove so different, that the method may need considerable modification. 



The following strata appear to be definitely distinguishable in the North Atlantic : — (a) a " superficial 

 stratum," of which the temperature varies with that of the atmosphere, and with the amount of insolation it 

 receives. The thickness of the stratum does not seem to be generally much above 100 fathoms ; and the greatest 

 amount of heating shows itself in the uppermost 50 fathoms. (6) Beneath this is an "upper stratum," the 

 temperature of which slowly diminishes as the depth increases down to several hundred fathoms ; the tempera- 

 ture of this stratum in high latitudes is considerably above the normal of the latitude ; but in the intertropical 

 region it seems to be considerably below the normal (c) Below this is a stratum in which the rate of diminu- 

 tion of temperature with increasing depth is rapid, often amounting to 10° or more in 200 fathoms, (d) The 

 whole of the deeper part of the North Atlantic, below 1000 fathoms, is believed to be occupied by water not 

 many degrees above 32°. With regard to this " glacial stratum," it is exceedingly important that its depth 

 and temperature should be carefully determined. 



It will probably be found sufficient in the first instance to take, with each deep bottom sounding, serial 

 soundings at every 250 fathoms, down to 1250 fathoms; and then to fill up the intervals in as much detail 

 as may seem desirable. Thus, where the fall is very small between one 250 and the next, or between any 

 one and the bottom, no intermediate observation will be needed : but where an abrupt difference of several 

 degrees shows itself, it should be ascertained by intermediate observations whether this difference is sudden 

 or gradual. 



The instrument devised by Mr. Siemens for the determination of submarine temperatures is peculiarly 

 adapted for serial measurements, as it does not require to be hauled up for each reading. It should, however, 

 be used in conjunction with the Miller-Casella thermometer, so as to ascertain how far the two instruments are 

 comparable : and this point having been settled, Mr. Siemens' instrument should be used in all serial soundings ; 

 and frequent readings should be taken with it, both in descending and ascending. 



A question raised by the observations of the U.S. Coast Surveyors in the Florida Channel, and by those of 

 our own surveyors in the China Sea, is the extent to which the colder and therefore heavier water may run up 



