612 
MR. J. PRESTWICH ON SUBMARINE TEMPERATURES. 
well amongst themselves to allow, with reasonable success, of the application of the 
same correction to all, excepting those of Dayman, and perhaps one or two others, which 
require larger corrections, and Wullerstorf’s, which are uncertain. 
With these few exceptions, and admitting slight qualifications for each particular case, 
the larger number of the early observations may, subject to a correction for pressure, be 
accepted as approximately accurate. The need of this correction for pressure was, as I 
have before observed, noted so early as 1823; but it was not until the voyage of ‘La 
Venus ’ that the necessary precautions were professedly taken against it, and that expe- 
riments were made to estimate its amount. Such estimates were then made by Du Petit- 
Thouars in tropical seas, subsequently by Martins and Bravais in arctic seas, and after- 
wards by Aime in an inland sea. The results of the several calculations are as follows : — 
Du Petit-Thouars made experiments with a protected and an unprotected thermo- 
meter at a depth of 1000 brasses or 1620 metres, which is equal to a pressure of 162 
atmospheres, and he was led to adopt a coefficient of 0 o, 01 Cent, per atmosphere as the 
measure of correction needed for unprotected thermometers. This gives 1° C. per 100 
atmospheres, or of 1°*8 F. per 3200 feet, or 1° Fahr. = 1780 feet. 
Ch. Martins concluded from his experiments, which were on a more limited depth, 
that a coefficient of 0°T3 Cent, per 100 metres, or of l o, 30 C. per 1000 metres* (equal 
to 2 0, 3 Fahr. per 3280 feet, or 1° for every 1426 feet), was required. 
Aime, again, from experiments in the Mediterranean with his special thermometro- 
graphs, came to the same conclusion as Du Petit-Tiiouars, viz. that for the pressure of 
every 100 atmospheres the instrument required a correction of about 1° Cent. 
These conclusions agree very closely with the more recent researches of Dr. Carpenter 
and the late Dr. Miller. The latter showedf that under a pressure of 2-^ tons (or 374 
atmospheres) per square inch, Six’s unprotected self-registering thermometers of three 
different constructions gave readings from 7°*5 to 10° Fahr. too high. Excluding the 
effects of the small amount of heat evolved from the water by compression (or some 
undetermined cause), which was found equal to 0 o, 9, the mean error ^of the three was 
8 0, 6 F. — 0 O- 9 = 7 O- 7 ; and, taking the pressure of one ton as equivalent to a depth of 800 
fathoms, this would be equal to a rise of 1° F. for every 1560 feet. But in those expe- 
riments one instrument (Six’s, with a spherical bulb) gave a variation of 2° in excess of 
the one with cylindrical bulb and of the Admiralty instrument. Now, as the two latter 
are of the forms almost always used, and Bunten’s instruments had also a cylindrical 
bulb, it is a question whether the one with spherical bulb should not be excluded. In 
that case the reading of the other two gives a mean of 8°F. — 0 o, 9 = 7°T as the error for 
pressure of 2-| tons, or equal to 1° Fahr. for every 1690 feet. 
It is true that considerable variation was found to exist in the effects of pressure on 
* M. Martins took the differences between each of the protected and unprotected “ thermometrograph.es,” 
and these he diminished in each case by 0°T, — “ quantite egale a la poussee de l’index.” 
t Proc. Roy. Soc. for 1869, vol. xvii. p. 485 : see also Proc. Roy. Soe. for 1870, vol. xviii. p. 409 ; and Com- 
mander Dayis, R.N., ibid. p. 347. 
