"I suppose that you did not, boys. But as we are talking of boats, pray can you think of any way of making a boat move through water without oars, or paddles, or sails, or something to pull it along?" "No indeed.--Oh yes, Uncle Philip, by steam." "Ah, I mentioned paddles, boys, and a steamboat is forced along by them." "No; Uncle Philip, we do not know." "Well, I will tell you then of another way in which I have no doubt a boat might be made to move. If there were any contrivance by which a large quantity of water could be kept in the boat, and if this water were forced out of tubes or holes at one end very violently, it would push against the water in which the boat was floating, and force her along. Some years ago a plan was thought of to make a steam engine throw the water out of the stern of the boat, and thus to force her along; and before that, Dr. Franklin tried some schemes for the same purpose, but they never succeeded. Now there is an insect which adopts precisely this plan, and perhaps some of those who thought of it got the notion from the insect." "What insect is it, Uncle Philip?" "It is the grub of the dragon-fly. If you catch one of these grubs and put it into a saucer of water with some of the dead leaves or sticks it had for a covering, you will see these leaves or sticks floating towards the tail of the grub, and afterward driven off again. This is because the insect is pumping in water, and then throwing it out. If you take one of them out of the water, and hold it with its head down, and let a drop of water fall upon its tail, it instantly sucks it in, and you can see it grow larger; and when it throws it out again it becomes smaller." "But, Uncle Philip, how can you see it suck the water in?" "Very easily, boys. When it is in the water, if you will colour some other water with indigo, or ink, or any thing else, and then hold a glass tube just over the tail of the insect, and very carefully put some of the coloured water into the tube, you will soon see the grub spirt out a stream of it to the distance of several inches: or if you will put the insect in a saucer of coloured water, and then suddenly move it, and put it into one of clean water, you will see it spirt out the coloured stream plainer still." "Why, Uncle Philip, it must have a pump inside of it." "It has, boys, something very like one. This stream of water is forced out to help the insect along; for though it has six feet, it uses them very little except for catching food. It drives the water out so strongly against the still water behind it, that it sends it forward, with a dart, very rapidly. Here are two pictures; one shows the pump open, and the other shows it shut." [Illustration] "Uncle Philip, is there any thing else curious about this insect?" "There is, boys, something well worth attention; did you ever see a mask?" "Do you mean, Uncle Philip, a face made of pasteboard, very frightful commonly, which you can wear over your own face?"