direction of a ball through the air, the distance it would reach to and the force with which it will strike any thing. All engineers, therefore, must be good mathematicians. Ch. But I think I have heard of gunners being little better than the common men. Fa. True-there is a way of doing that business, as well as many others, by mere practice; and an uneducated man may acquire skill in pointing a cannon, as well as in shooting with a common gun. But this is only in ordinary cases, and an abler head is required to direct. Well-now suppose your ship completely fitted out for sea, and the wind blowing fair; how will you navigate her? Ch. I would spread the sails, and steer bythe rudder. Fa. Very well-but how would you find your way to the port you were bound for? Ch. That I cannot tell. Fa. Nor perhaps can I make you exactly comprehend it; but I can show you enough to convince you that it is an affair that requires much knowledge, and early study. In former times when a vessel left the sight of land, it was steered by observation of the sun by day, and the moon and stars by night. The sun, you know, rises in the east, and sets in the west: and at noon, in these parts of the world, it is exactly south of us. These points, therefore, may be found out when the sun shines. moon and stars vary; however their place in the sky may be known by exact observation. Then, there is one star that always points to the north pole, and is therefore called the pole-star. This was of great use in navigation, and the word pole-star is often used by the The poets to signify a sure guide. Do you recollect the description in Homer's Odyssey, when Ulysses sails away by himself from the island of Calypso, how he steers by the stars? Ch. I think I remember the lines in Pope's translation. Fa. Repeat them, then. Ch. Placed at the helm he sat, and marked the skies, Nor clos'd in sleep his ever-watchful eyes; There view'd the Pleiads, and the Northern Team, And great Orion's more refulgent beam, To which, around the axle of the sky, The Bear revolving points his golden eye; Fa. Very well-they are fine lines indeed! You see then, how long ago sailors thought it necessary to study astronomy. But as it frequently happens, especially in stormy weather, that the stars are not to be seen, this method was subject to great uncertainty, which rendered it dangerous to undertake distant voyages. At length, near 500 years since, a property was discovered in a mineral, called the magnet or loadstone, which removed the difficulty. This was, its polarity, or quality of always pointing to the poles of the earth, that is, due north and south. This it can communicate to any piece of iron; so that a needle well rubbed in a particular manner by a loadstone, and then balanced upon its centre so as to turn round freely, will always point to the north. With an instrument called a mariner's compass, made of one of these needles, and a card marked with all the points north, south, east, west, and the divisions between these, a ship may be steered to any part of the globe. In Ch. It is a very easy matter, then. Fa. Not quite so easy, neither. a long voyage, cross or contrary winds blow a ship out of her direct course, so that, without nice calculations both of the straight track she has gone, and all the deviations from it, the sailors would not know where they were, nor to what point to steer. It is also frequently necessary to take observations, as they call it; that is, to observe with an instrument where the sun's place in the sky is at noon, by which they can determine the latitude they are in. Other observations are necessary to determine their longitude. What these mean, I can show you upon the globe. It is enough now to say that, by means of both together, they can tell the exact spot they are on at any time; and then, by consulting their map, and setting their compass, they can steer right to the place they want. But all this requires a very exáct knowledge of astronomy, the use of the globes, mathematics, and arithmetic, which you may suppose is not to be |