I glanced at the instrument, which, together with a tell-tale compass, swung from the skylight transoms, and saw that the mercury had sunk in the tube to the extent of nearly an inch since the last setting of the vernier; and, as it was our custom in the Slave Squadron at that time to set the instrument at 8 o'clock a.m. and 8 o'clock p.m., it meant that the mercury had fallen to that extent during the night!  What was about to happen?  I had observed nothing portentous in the aspect of the weather, while on deck, unless, indeed, the softening away of the trade- wind and the hazy condition of the atmosphere might be regarded as portents.  Yet that could hardly be, for I had observed the same phenomena before, yet nothing particular had come of it.  I decided to have a talk with Tasker, the gunner's mate, and get his views on the matter; he was a man of very considerable experience, having been a sailor before I was born; I therefore at once entered my cabin, and proceeded to dress; after which I returned to the deck, where Tasker was officer of the watch.  I found him sitting aft on the stern grating, replacing his socks and shoes, which he had removed from his feet at four bells in order to take a leading part in the matutinal ceremony of washing decks.  I had already seen him a little earlier that morning, and exchanged greetings with him; I therefore at once, and without any circumlocution, plunged into the subject by asking: