Page:Proceedings of the Royal Society of London Vol 2.djvu/106

Rh arrival at Ceylon; and the results lead him to adopt the opinion that the ocean resembles the atmosphere in being, cæteris paribus, nearly of the same specific gravity throughout. The water used in the experiment was always taken from the surface of the ocean. The variation of specific gravity was most observable when the water was rough and agitated, and seemed in one instance diminished by heavy rain.

Dr. Davy doubts whether a modern traveller of high authority is correct in supposing that a peculiar specific gravity belongs to the water of each zone; for in his own experiments, the water taken in latitude 0° 12′ S., and 22° 36′ S., was of similar specific gravity, as also that taken at 34° 25′ S., and that washing the shores of Columbo.

The trials of the temperature of the air and water were, during the greater part of the voyage, made every two hours, night and day. The variations of atmospheric temperature, says the author, follow the course of the sun. They are pretty considerable whilst he is above the horizon, and very insignificant during the night. At a great distance from land, and with a steady wind between and bordering upon the tropics, the diurnal variation of atmospheric temperature appeared perfectly regular; its maximum precisely at noon, its minimum towards sunrise. In a calm, the maximum of heat was some time after noon, and the regular law of variation is more obviously interfered with by storms and rain.

The temperature of the sea was found liable to variations nearly as great as those of the incumbent atmosphere. In fine quiet weather, at a great distance from land, the maximum of temperature was about 3, and the minimum towards sunrise. It is, however, subject to irregularities. In tempestuous weather superficial currents seem to be established in the direction of the prevailing winds, which increase or lower the temperature according as the wind is hot or cold. Where the sea is shallow, its temperature is comparatively low; a fact which may sometimes prove useful in indicating to the mariner the vicinity of shallows. In approaching the Cape of Good Hope and Ceylon, the author had occasion to observe this fact: in the latter case there was a reduction of 2 on coming into soundings. In considering the effects of currents upon the temperature of the sea, Dr. Davy particularly notices that which flows round the bank of Lagullas from the S.E. coast of Africa, and which is 10 above the surrounding sea; a difference partly referable to the banks which border the current. The dense mist which occasionally covers the Table Mountain is considered by Dr. Davy as connected with this current, and produced by the condensation of the vapour rising from this current by a cold S.E. breeze, during which the phenomenon only happens.

This communication concludes with some general and practical inferences connected with the use of the thermometer at sea: it contains several tables of results, and of meteorological observations.