MR. J. PRESTWICH ON SUBMARINE TEMPERATURES. 
613 
some other instruments ; but with the care taken in the construction of our best ther- 
mometers, and of those of Buntef, which were generally employed, the chances of 
greater variation than that here indicated are reduced to a minimum*. 
The foregoing estimates show that with good instruments the effect of pressure equals 
an increase of about 1°F. for every 1400 to 1800 feet of depth ; and in adopting a coeffi- 
cient of 1° F. for every 1700 feet as the necessary correction of all the observations in the 
Tables, excepting those made with protected instruments or corrected by the original 
observer, and excepting also those before named as requiring larger corrections in con- 
sequence of using unfit or unsuitable instruments or instruments of a different class, I 
feel that I am below rather than above the true measure of allowance. 
§ III. Summary of the preceding Observations. 
Although the early observers noted the decrease of temperature with the increase of 
depth, it was not until 1823-26 that Lenz proved that this decrease held good to the 
greater depths of temperate and tropical seas, and that the water at depths in the open 
oceans was but little above the zero of Centigrade. 
The substitution of the self-registering thermometer for the older methods led for a 
time, owing to the neglected error of pressure, to a retrograde course ; for the voyages 
of Beechey, Kellett, and others which followed between 1826 and 1836, while they 
added largely to the number of observations at greater depths, gave, in so doing, 
increased importance to the error, from the circumstance that the pressure on the 
instrument not only counterbalanced the effect of the greater cold at increased depths, 
but often gave readings (uncorrected) somewhat higher at those depths than at lesser 
ones. From this cause, and from inattention to the different properties of sea- and 
fresh water, an erroneous conclusion was drawn from observations otherwise valuable, 
which for a time greatly retarded the progress of ocean physics. 
The first to fall into this error was D’Urville, who, misled by the coincidence of 
temperature obtained by him in some of his deepest soundings, and of the nearly like 
minimum temperature (4° to 5° C.) so frequently recorded (with his unprotected ther- 
mometers) by Beechey and others at greater depths, concluded, in ignorance apparently 
of Lenz’s observations, that this uniformity of temperature was the result of a general 
* With, respect to these variations, Dr. Carpenter, after speaking of the results obtained on the ‘ Porcupine ’ 
expedition with the Mtller-Casella instrument, observes: — “With these results, obtained with thermometers 
upon which complete reliance can be placed, those obtained last year with the best ordinary thermometers are 
found to be in close accordance, when the proper correction for pressure is applied to them.” He then instances 
two cases in which experiments were made on both expeditions at nearly the same places and in nearly 
similar depths. In one case, at a depth of 550 fathoms, the difference exceeded the estimate by about 1°, 
in the other, at a depth of 550 fathoms, it amounted to 2°-2 E., or was “ exactly equivalent to the correc- 
tion for pressure at that depth in the unprotected thermometers.” Dr. Carpenter concludes : — “ This very 
near accordance gave us, of course, a feeling of great satisfaction in our last year’s work; and it fully justified 
our conclusion that, whatever might be the pressure-correction required by the instruments then employed, it 
would not affect the differences obtained at nearly approximating depths.” (Proc. Roy. Soc. vol. xviii. p. 455.) 
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