Page:Journal of the Royal Geographical Society of London, Volume 1 (2nd edition).djvu/241

Rh .027; and the mean periodical oscillation for 3. − 015, 6 − 005, 9  + 006, noon + 005, 3  − O25, 6  − 004, 9  + 018, and for midnight, + 015. It is hardly necessary to dwell upon the dates given for the mean altitude of the barometer in different latitudes: as compared with one another the results are useful; but the true height of the column of mercury in any given place can only be obtained from the annual mean, which, at the level of the sea and at the same temperature, is everywhere pretty nearly the same. The highest range of the barometer in the polar regions observed by Captain Sir E. Parry was 30·86, which it attained in Melville Island, on the 27th of April, 1820; and Captain Beechey observed the barometer at 30·32 in Kotzebue Sound, on September 1st, 1826.

The tables of the observed temperature and humidity of the air in different parallels, and which have been used in the correction of errors, are minute, and extremely useful for reference.

The practical application of observations on the temperature of the sea have, as far as we know, been only twofold. The celebrated Franklin first fixed the attention of natural philosophers on the phenomena which are presented in the temperature of the ocean above shallows, and his remarks have since been verified by De Hnmboldt. Captain Beechey mentions in his narrative some remarkable cases where changes in the temperature of the surface of the sea were forerunners of shifts of wind, which he thinks they preceded, even before any change in the temperature of the air. The observations on the temperature of the surface of the sea in the Appendix of Captain Beechey's work were made every four hours, by plunging a thermometer into a bucket of sea water immediately it was drawn up, and they are compared with the mean of bi-horal observations on the temperature of the air. The mean only of the daily observations being given, we cannot ascertain how far they verify Péron's notions on the variations between mid-day and midnight, and the relation of these to the temperature of the ambient air at the same period; but the observations on the temperature of the surface of the sea, compared with that of the atmosphere, appear to demonstrate that, in the same situation, the former is greater than the latter, and, consequently, that the law in hydrography is correct, which establishes that the mean term of the temperature of the waters of the ocean at their surface, and far from the continents, is greater than that of the atmosphere with which the waters are in contact. This is more particularly true with regard to the equatorial and inter-tropical regions, as also to seas distant from land, for exceptions were met with in Kotzebue Sound and in other places. Observations on the temperature of the sea at different depths are perhaps of still greater interest, both on account of the difficulty attendant on experiments of that