74 



arrival at Ceylon ; and the results lead him to adopt the opinion that 

 the ocean resembles the atmosphere in being, ceeteris paribus, nearly 

 of the same specific gravity throughout. The water used in the ex- 

 periment was always taken from the surface of the ocean. The vari- 

 ation 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 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 Co- 

 lumbo. 



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 bor- 

 dering upon the tropics, the diurnal variation of atmospheric tempe- 

 rature 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 ob- 

 viously 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 P. M., 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 in- 

 crease 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 con- 

 sidering the effects of currents upon the temperature of the sea, Dr. 

 Davy particularly notices that which flows round the bank of Lagul- 

 las from the S.E. coast of Africa, and which is 10 above the surround- 

 ing sea ; a difference partly referable to the banks which border the 

 current. The dense mist which occasionally covers the Table Moun- 

 tain 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 in- 

 ferences connected with the use of the thermometer at sea : it con- 

 tains several tables of results, and of meteorological observations. 



