The explanation is this: On the first night of our watch, a few hours after having taken my station in the tree, a fog had suddenly arisen upon the rivers, shrouding the channels of both. It was the _white fog_--a well-known phenomenon of the Mississippi--that often extends its dangerous drapery over the bosom of the "Father of Waters:" a thing of dread, even to the skilled pilots who navigate this mighty stream. On that particular night, the fog lay low upon the water: so that in my position near the top of the tree I was entirely clear of its vapoury disc; and could look down upon its soft filmy cumuli floating gently over the surface--white and luminous under the silvery moonlight. The moon was still shining brightly; and both sky and forest could be seen as clearly as ever. The water-surface alone was hidden from my sight-- the very thing I was most anxious to observe. As if by some envious demon of the flood, this curtain seemed to have been drawn: for, just as the fog had fairly unfurled itself, I fancied I could hear the dipping of a paddle at no great distance off in the channel of the stream. Moreover, gazing intently into the mist--as yet thin and filmy--I fancied I saw a long dark object upon the surface, with the silhouettes of human forms outlined above it--just as of a canoe _en profile_ with passengers in it. I even noted the number of the upright forms: three of them--which exactly corresponded to that of the party we were expecting. So certain was I at the moment, of seeing all this, that I need not have shouted to assure myself. Excited with over-eagerness, I did so; and hailed the canoe in hopes of obtaining an answer. My summons produced not the desired effect. On the contrary, it seemed to still the slight plashing I had heard; and, before the echoes of my voice died upon the air, the dark objects had glided out of sight-- having passed under thick masses of the floating vapour. Over and over, I repeated my summons--each time changing the form of speech, and each time with like fruitless effect! The only answer I received was from the blue heron, that, startled by my shouts, rose screaming out of the fog, and flapped her broad wings close to my perch upon the tree. Whether the forms I had seen were real--or only apparitions conjured up by my excited brain--they vouchsafed no reply; and, in truth, in the very next moment, I inclined to the belief that my senses had been deceiving me!