ME. J. PEESTWICH ON SUBMAEINE TEMPEEATUEES. 
605 
He then wrote to England for stronger registering thermometers, which were sent to 
him in Australia, but of which he gives no particulars further than stating that they 
were stronger. Before receiving these, he apparently renewed his observations with 
instruments obtained in Australia. Consequently it is probable that each of these sets 
of instruments were of different construction, and may require a different correction — 
those used during the first part of the voyage a larger correction than those used during 
the latter period. In the absence of sufficient information this cannot be attempted ; 
and the general formula given further on has been applied to the correction of all his 
observations I have had occasion to use in the construction of the sections. 
With regard to the observations themselves, they may be also sometimes open to 
objection in consequence of the great difficulties under which they were so constantly 
taken. The severe cold, the inclemency of the weather, and the tediousness of the 
operation are all elements of possible error to be taken into account. The one cause 
may have led at times to the shifting of the index, and the other to some want of accu- 
racy in the reading ; for I cannot conceive it possible for any set of thermometers to have 
recorded, in the innumerable cases mentioned, the same one and uniform temperature 
of 39°‘5 at and beyond a certain depth. Even supposing a uniform temperature of that 
exact degree did exist at certain determined depths, it is in the highest degree impro- 
bable that any instruments would give the exact same record. There is not only the 
risk of shifting of the index, but there is the certainty that the ordinary imperfection 
and variation of the instruments would most certainly prevent it. With the greatest 
care and with standard instruments especially selected, MM. Martins and Bravais, out 
of ten sets of observations each made with two, three, or four thermometers, only give 
one instance in which the readings of two of them agree. In the other cases they 
differ from 0°T to 1° Fahr. 
Nevertheless, apart from this point, and supposing them to be approximately correct, 
the observations of Sir James Boss are, from their number, depth, and position, very 
valuable, and, subject to correction, they furnish fairly available results, although, 
from the cause before mentioned, it may not be certain whether the correction applied 
gives the true reading in all cases within one, two degrees, or in some cases possibly 
more. Owing also to this use of unprotected instruments Boss came to the same con- 
clusion as D’Urville with respect to the existence of a zone of a uniform surface- 
temperature in given latitudes, and likewise with respect to the persistence of the same 
uniform temperature of 39 0, 5 Fahr. at given depths in the great oceans. In this opinion 
he seems to have been biassed, similarly with his predecessors, by the belief that the 
density of sea-water was, like that of fresh water, greatest at that temperature. 
In 1840-44 M. Aime made a series of important observations on the temperature of 
the air and sea between Marseilles and Algiers*. The experiments, which were carried 
on for a series of years, proved that the diurnal variations of temperature in the Medi- 
* “Memoir© sur la temperature de la Mediterranee,” Annales de Cliimie et de Physique, 1845, 3 me ser. 
vol. xt. p. 1 ; and Comptes Eendus for Sept. 1844. 
4 m 2 
