s4 THE ATLANTIC. (CHAP, 1, 
employment of the Miller-Casella thermometer. They will be special- 
ly applicable to the area in which the work of the expedition will com- 
mence; but the thermal conditions of other areas may prove so differ- 
ent, that the method may need considerable modification. 
The following strata appear to be definitely distinguishable in the 
North Atlantic: (@) A “superficial stratum,” of which the temperature 
varies with that of the atmosphere, and with the amount of insolation 
it receives. The thickness of this stratum does not seem to be general- 
ly much above 100 fathoms; and the greatest amount of heating shows 
itself in the uppermost fifty fathoms. (6) Beneath this is an “ upper 
stratum,” the temperature of which slowly diminishes as the depth in- 
creases down to several hundred fathoms: the temperature of this stra- 
tum in high latitudes is considerably above the normal of the latitude ; 
but in the intertropical region it seems to be considerably delow the nor- 
mal. (c) Below this is a stratum in which the rate of diminution of 
temperature with increasing depth is rapid, often amounting to 10° or 
more in 200 fathoms. (@) 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 care- 
fully determined. 
It will probably be found sufficient in the first instance to take, with 
each deep dottom sounding, seria? 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 intermedi- 
ate observation will be needed; but where an abrupt difference of sev- 
eral degrees shows itself, it should be ascertained by intermediate ob- 
servations whether this difference is sudden or gradual. 
The instrument devised by Mr. Siemens for the determination of sub- 
marine 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 as- 
certain how far the two instruments are comparable: and this point 
having been settled, Mr. Siemens’s instrument should be used in all se 
rial 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 
