in search of that from which he was daily becoming more and more distant. And supposing all these difficulties to have been avoided, there was still the increase of distance requisite on sailing along the two sides of a right-angled triangle, instead of taking the diagonal, like a person walking round the two sides of a square to arrive at any particular house, instead of crossing it obliquely.

We will, however, defer the consideration of this second difficulty to the next chapter, premising only that, although the practical attainment of longitude has been beset with many difficulties, and is still much more complicated than latitude, yet its theory is very simple, and, we think, will prove a very attractive subject.

CHAPTER IX.

NAVIGATION CONTINUED LONGITUDE.

"Rude as their ships was navigation then,

No useful compass or meridian known;

Coasting, they kept the land within their ken,

And knew no north but when the pole-star shone."

DRYDEN.

LONGITUDE, as we stated in introducing the subject of navigation in the last chapter, is denoted by ideal lines intersecting the earth from pole to pole, resembling the sections of an orange when cut from the crown to the stalk. In England, longitude is reckoned either as east or west of Greenwich; that is, the first of these ideal lines, from the north to the south pole, passes through the Royal Observatory at Greenwich. Any place to the eastward of it is described as so much east longitude, or if to the west, as west longitude; whereas, if it is directly north or south of Greenwich, it would be called on the meridian of Greenwich. Longitude consists, therefore,

in our different situations as regards east and west; and this difference is rendered perceptible to us by the different hours at which the various meridians come under the sun.

The earth revolving on its axis or poles brings the same place under the sun every twenty-four hours, causing by its revolutions our days and nights; now as twenty-four hours give the whole circumference of the globe, or 360 degrees, every hour will give one twenty-fourth part of it, or 15 degrees; and so on for every division of time.

When any meridian comes under the sun, it denotes noon, or twelve o'clock; consequently, if we possess a watch regulated to Greenwich time, and find that when the sun indicates noon, or twelve o'clock, in the place where we are, that the watch shews one o'clock, it is evident that we are an hour to the westward of Greenwich, the sun having passed Greenwich meridian one hour before he arrived at ours we are therefore 15 degrees, or one twenty-fourth part of the way westward, round the globe.

To illustrate this, we will refer to the little diagram in the frontispiece. The movable circle represents the northern hemisphere of the globe, the circumference forming the equator; the circles, parallels of latitude; the radiating lines, meridians of longitude; the dark line below, the

twelve o'clock line, being the direction in which the sun is supposed to shine: consequently, when any part of the movable circle is brought opposite to this line, it is noon, or twelve o'clock, to that place. The figures round the outside denote Greenwich time, and may be considered as a clock regulated to Greenwich time, of which the line marked " Meridian of Greenwich" on the movable circle may be considered as the hour-hand or index, because whatever figure it points to represents Greenwich time, when the world is in the position with regard to the sun represented by the diagram.

Let us now turn the circle till we bring New York under the sun; it is then noon, or twelve o'clock, at New York. But what is it by Greenwich time? By looking at the Greenwich clock on the outside circle, we find the hand, or line marked " Meridian of Greenwich," pointing to five, indicating that it is five o'clock at Greenwich; consequently, there are five hours' difference between that time and the time at New York, or five hours' difference of longitude. Now, as in each hour the earth revolves 15 degrees, five hours will give 75 degrees, or rather more than one-fifth part round the world. Thus the sun may be considered as the hand, which, pointing to the earth as it revolves, marks out the different degrees of longitude, whilst the clock is the interpreter which indicates what those degrees

are.

At noon, by the dial, it tells us the sun points to Greenwich; and every place on that meridian, at one o'clock, to a meridian removed 15 degrees to the west of it; at five, to New York; at midnight, to our antipodes; at six in the morning, to India; and so on till noon again. A navigator, therefore, if furnished with a chronometer, or watch, on which he can depend, and having regulated it to Greenwich time before leaving the Thames, will have little difficulty, at any time during his voyage, in determining his longitude: but as instruments of this description are ever liable to error and casualty, he turns for a substitute to the unerring hand of Nature, and makes the starry vault of heaven his clock.

Had there been some phenomenon which took place at stated intervals, such as the extinction and re-appearance of some particular star, or were the eclipses of the sun and moon of daily occurrence, knowing, by calculation, the moment they would happen by Greenwich time, they would answer the purpose of the clock in the diagram; but such not being the case, and the eclipses of the satellites or moons of Jupiter, though of frequent occurrence, being too minute for common observation, astronomers have sought for other means, and the moon, which, from her near proximity to us, alters her apparent position very rapidly amongst the heavenly

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