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NATURE. 
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[Fan. 9, 1873 
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’s instrument should be used in all serial soundings ; 
and fceq1eat readings should be taken with it, both in descend- 
ing and ascending. : 
A question raised by the observations of the U.S. Coast Sur- 
veyors 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 wf /il/ on the sides of declivities. The 
position of the Azores will probably be found very suitable for 
observations of this kind. Temperature-soundings should be 
taken at various depths, especially on their north and south 
slopes, and in the channels between the Islands ; and the tem- 
peratures at various depths should be compared with those of 
corresponding depths in the open ocean. 
It is in the southern oceans that the study of ocean-tempera- 
tures at different depths is expected to afford the most important 
results ; and it should there be systematically prosecuted. The 
great ice-barrier should be approached as nearly as may be 
deemed suitable, in a meridian nearly corresponding to the centre 
of one of the three great southern oceans ;—say to the south of 
Kerguelen’s Land ; and a line of soundings should be carried 
north and south as nearly as may be. 
In connection with the limitation of the area and depth of the 
reef-building corals, it will be very important to ascertain the 
rate of reduction of temperature from the surface downwards in 
the region of their greatest activity; as it has been suggested 
that the limitation of living reef-builders to 20 fathoms may be a 
thermal one. 
Wherever any anomaly of temperature presents itself, the con- 
dition of such anomaly should, if possible, be ascertained. Thus 
there is reason to believe that the cause of the temperature of the 
surface-water being below that of the sub-surface stratum, in the 
neighbourhood of melting ice, is that the water cooled by the ice, 
by admixture with the water derived from its liquefaction, is also 
rendered less salt, and therefore floats upon the warmer and 
salter water beneath. Here the determination of Specific Gra- 
vities will afford the clue. In other instances a warm current 
may be found beneath a colder stratum; and the use of the 
* current-drag” might show its direction and rate, In other 
cases, again, it may happen that a warm submarine spring is 
discharging itself, —as is known to occur near the island of As- 
cension. Jn such a case, it would be desirable to trace it as 
nearly as may be to its source, and to ascertain its composition. 
Movements of the Ocean.—The determination of Swrface- 
Currents will, of course, be a part of the regular routine, but it 
is particularly desi-able that accurate observations should be 
made along the line of sounding in the Southern Ocean, as to 
the existence of what has been described as a general “ Southerly 
set” of Oceanic water, the rate of which is probably very slow. 
It is also very important that endeavours should be made to test 
by the ‘‘ current-drag,” whether any wxderflow can be shown to 
exist from either Polar basin towards the Equatorial region. A 
suitable locality for such experiments in the North Atlantic would 
probably be the neighbourhood of the Azores, which are in the 
line of the glacial flow from the North Polar Channel. The 
guide to the depth at which the current-drag should be sus- 
pended, will be furnished by the thermometer, especially where 
there is any abrupt transition between one stratum and another. 
It would be desirable that not only the rate and direction of 
surface-drift, but those of the subsurface-stratum at (say) 200 
fathoms’ depth, should be determined at the same time with 
those of the deep stratum. 
Tidal Observations.—No opportunity of making tidal obser- 
vations should be lost. Careful observations made by aid of a 
properly placed tide-pole in any part of the world will be valu- 
able. Accurate measurements of the sea-level once every hour 
(best every Zwzar hour, z.e. at intervals of 14 2™ of solar time) 
for a lunar fortnight (the time of course being kept) would be 
very valuable information. 
Bench-marks.—In reference to the interesting question of the 
eJevation or subsidence of land, it will be very desirable, when 
sufficient tidal observations can be ob’ained to settle the mean 
level of the sea, that permanent bench-marks should be esta- 
blished, recording the date and height above such mean level. 
Even recording the height to which the tide rose ona certain 
day and time, would render a comparison possible in future 
years. 
A good determination of the mean sea-level by the simple 
operation of taking means may be made, in less than two days, 
with even a moderate number of observations properly distributed 
so as to subdivide both sclar and lunar days into not less than three 
equal parts. 
both solar and lunar. Take a lunar day at 24 48™ solar time, 
which is near enough, and is convenient for division ; and choos- 
ing any convenient hour for commencement, let the height of the 
water be observed at the following times, reckoned from the com- 
mencement :— 
ih an. h m h m 
oo 8 o 16 0 
8 16 16 16 24 16 
16 32 24 32 32 32 
The observations may be regarded as forming three groups of 
three each, the members of each group being seprated by 8 
hours solar or lunar, while one group is separated from the next 
by 8 hours lunar or solar. In the mean of the nine results the 
lunar and solar semi-diurnal and diurnal inequalities are all four 
eliminated. 
Nine is the smallest number of observations which can form a 
complete series. If the solar day be divided into m and the lunar 
into 2 equal parts, where and 2 must both be greater than 2, 
there will be 72 observations in the series; and if eithe- # or 
mn be a multiple of 3, or of alarger number, the whole series may 
be divided into two or more series having no observation in 
common, and each complete in itself. The accuracy of the 
method can thus be tested, by comparing the means obtained 
from the separate sub-series of which the whole is made up. 
Should the ship’s stay not permit of the employment of the 
above method, a very fair determination may be made in less 
than a day, by taking the mean of 7 observations taken at inter- 
vals of the 7th part of a lunar day, being greater than 2. Thus 
if 7=3, these observations require a total interval of time 
amounting to only 16 32™. The theoretical error of this method 
is very small, and the result thus obtained is decidedly to be pre- 
ferred to the mere mean of the heights at high and low water. 
The mean level thus determined is subject to meteorological 
influences, and it would be desirable, should there be an oppor- 
tunity, to redetermine it at the same place at a different time of, . 
year. Should a regular series of observations for a fortnight be 
instituted, it would be superfluous to make an independent deter- 
mination of the mean sea-level by either of the above methods at 
the same time. 
Besides taking observations on the ordinary waves of the sea 
when at all remarkable, the scientific staff should carefully note 
circumstances of any waves attributable to earthquakes. 
Specific Gravity.—The Specific Gravity of the surface and 
botiom-water should be carefully compared, whenever soundings 
are taken ; and whenever Serial Soundings are taken, the Specific 
Gravity at intermediate depths should be ascertained. Every 
determination of specific gravity should be made with careful 
attention to temperature ; and the requisite correction should be 
applied from the best Table for its reduction to the uniform 
standard of 60°. It would be well to check the most important 
results by the balance ; samples being preserved for examination 
in harbour. Wherever the temperature of the surface is high— 
especially, of course, in the intertropical region-- samples should 
be collected at every 10 fathoms for the purpose of ascertaining 
whether any effect is produced upon the specific gravity of the 
upper stratum by evaporation, and how far down this effect 
extends. 
Transparency of the Water.—Observations for transparency 
should be taken at various depths and under different conditions 
by means of Mr. Siemens’s photographic apparatus. As, how- 
ever, the action of this depends upon the more refrangible rays, 
and the absorption of these and of the more luminous rays might 
be different, and that in a manner varying with circumstances, 
such as the presence or absence of suspended matter, &c., the 
transparency of the sea should also be tested by lowering a white 
plate or large white tile to various measured depths, and noting 
the change of intensity and colour as it descends, and the depth 
at which it ceases to be visible. The state of the sky at the time 
should be mentioned, and the altitude of the sun, if shining, 
roughly measured, or if not shining, deduced from the time 
of day. 
Relation of Barometric Pressure to Latitude—In Poggendorff’s. 
*“Annalen,” vol. xxvi. 1832, p. 395, is a remarkable paper 
by Prof. G. F. Schouw on the relation between the height of the 
barometer at the level of sea, and the latitude of the place of 
“ots 
Suppose, for example, we choose 8-hour intervals, . 
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