3rd January 1836

Captain Fitzroy’s Journal
To the succeeding day at noon we were set only seven miles, by the water, and that due east. Afterwards, in our passage to Port Jackson, we had alternately northerly and south-easterly currents of about ten miles a day, and it was easy to tell which current we were in, by the temperature of the sea:—while the stream set from the north, the water thermometer showed about 72°; but when the current was running from the southward, the temperature of the ocean, a foot below, as well as at, the surface, was only 67°. I ought to have remarked elsewhere, if I have not already done so, that the thermometer may be used at sea to detect and trace currents; but little, if any, confidence can be placed in its indications as a guide to the approach of land. Icebergs may indeed affect it, but they will affect the temperature of the air probably sooner than that of the ocean.