"I have," answered Leo Pownall, whose father owned a saw-mill. "I have towed lots of them on the mill-pond." "To which end of the log did you make fast?" inquired the captain. "To either end; just as it came handy," replied the student. "Then you sometimes did more work than was necessary with your oars. A log tows easier when you make fast to the big end," continued the principal, waiting for the pupils to digest the idea. "I don't see what difference it can make," added Leo. "If anything, I should say that the small end would open a passage through the water more readily than the big end." "I suppose none of you ever saw a whale, but most of you have caught horn-pouts, or bullheads." "I have seen a whale on exhibition in New York," interposed Luke Bennington. "What was the shape of his head?" "The one I saw was round; but I have seen pictures of whales in which the head was nearly square." "How is it with the pout!" "His mouth is about the widest part of him," laughed Alick Hartford. "Take fishes in general, in what part of the body do you find the greatest girt?" asked the principal. "Just astern of the head," replied Kit Burlington. "In some fishes, about one third of the length from the mouth," added Bark Duxbury. "Very good; you are about right, though some fishes vary from the general rule. Now don't you think Nature made a mistake, Leo Pownall, and that fishes ought to swim tail first instead of head first, as you would tow a log?" "I suppose God made the fishes all right; but He gave some of them very sharp noses," returned the saw-mill owner's son. "Corresponding to the shape of the butt of a log after it has been felled; but the greatest girt is still near the head. This is the general shape of the hull of a vessel." "But the head of a sperm whale is almost square; and no other fish is like him," added Phil Gawner. "The whale is not a fish, Gawner. I have seen a school of porpoises alongside an ocean steamer. Their greatest girt is one third of the length from the head end; but they will swim past a fast steamer, and make something like twenty knots an hour," said Captain Gildrock. "I was trying to find the porpoise in Wood's Natural History the other day; but there is no such fish in the book," added Sol Guilford. "Where did you look?" "In the volume about fishes." "The porpoise is not a fish, and you would have found it in the volume marked 'Mammalia,'" replied the principal with a smile. "But isn't the porpoise a fish? He lives in the water." "So do hippopotami; but they are not fishes. Whales, porpoises, dolphins, seals, and some others, are mammals; that is, they suckle their young as a cow does a calf. Properly they are not fishes, though they are very often called so."