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we read presents us with fresh occasions of wonder and delight.

We left Galileo looking through his telescope at the changing aspect of Venus, and he might previously have imagined that such an alteration in her appearance must take place; but when the astronomer directed his tube towards Jupiter, he saw that of which he could have had no conception: he perceived this noble planet to be accompanied by four moons, which revolve round him at different distances, and in different periods of time. Nor was this a mere spectacle to be admired, without leading to any important result. The eclipses of the moons of Jupiter have enabled astronomers to determine the longitude of different places on the earth.

Perhaps I may be able to give you some notion of the way in which this is done. No doubt some of you have learned, while studying your problems on the globe, that every hour answers to fifteen degrees of the equator. If you look at the map of Europe, you will see that the first meridian-that from which we

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reckon our degrees of longitude-passes over London; (to speak quite correctly, I should say over the Royal Observatory, in Greenwich Park;) and that the fifteenth degree passes over Mount Etna. Now suppose the sun to be exactly south of Etna; it is then noon-day to the inhabitants of that mountain, and, at the same moment, it is eleven o'clock in the morning to the astronomer at Greenwich. The earth must continue turning on its axis for one hour, before the Observatory at Greenwich will be exactly opposite to the sun:-it will then be noon at Greenwich; and Mount Etna having also passed on towards the east, the sun will begin to shine on its western side:-it is then one in the afternoon to the inhabitants of Etna.

We will now, if you please, suppose that one of the satellites of Jupiter is going to be eclipsed, and that two astronomers, one at Greenwich, and the other on Mount Etna, have their telescopes in readiness to observe the phenomenon. They will both see the eclipse at the same moment of time; but if the English

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observer finds, on looking at his watch, that it is seven o'clock in the evening, the watch of the Sicilian will show him that it is eight o'clock. This difference of an hour in the watches, which we suppose to be equally well regulated, shows that there are fifteen degrees of longitude between the two places.

As

Perhaps you may say that Greenwich and Mount Etna are so widely distant, that neither of the observers could know what the other was doing. They might, however, have agreed to make such an experiment, because the time of eclipses may be calculated beforehand. tronomers have observed these satellites so accurately, that they can tell how long they are in revolving round Jupiter, when they will pass into his shadow, and how soon they will reappear on the opposite side of that beautiful planet. You know that in a common almanac, the time of the rising and setting of the sun for every day in the year is carefully noted; and thus, in the Nautical Almanac, there are printed tables, showing the time of these eclipses.

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A person possessing this book, a good telescope, and a well regulated watch, in whatever part

of the world he happens to be, may direct his telescope to Jupiter, and observe the difference of time between his watch, at the moment of the eclipse, and the hour and the minute in the printed table, which is computed for the meridian of Greenwich: he must then allow fifteen degrees for every hour of difference; and thus he may easily find the longitude of any place on land. Unfortunately for sailors, who stand most in need of such knowledge, the rolling of the ship often makes it impossible to fix a telescope properly. If the satellites of Jupiter could be seen distinctly with the naked eye, the longitude might be found by a common sailor, with nearly as much ease and certainty, as the latitude; but as no method has yet been devised for rendering the telescope independent of the motion of the vessel, they are obliged to have recourse to other means.

An eclipse of the moon, like those of the satellites of Jupiter, may be seen from every

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part of the earth, where the moon is above the horizon, and seen with the naked eye. This, then, seems the very thing that is wanted for the sailor; he may find in his Nautical Almanac, the hour and minute of the eclipse at Greenwich, and turn the difference of time into degrees of longitude. It seems so; but this method does not fully answer the sailor's purpose. Eclipses of the moon happen too seldom to be of much service; and when they do happen, the beginning and end of the eclipse cannot always be so clearly distinguished as to prevent error in observing the time, which may occasion error in reckoning the longitude. The satellites of Jupiter, on the contrary, are so quick in their movements, and pass so often into the shadow of the planet, that there is scarcely a night in which one or other of them is not eclipsed. Beside this advantage, they enter the shadow, and emerge from it so suddenly, that the time of the phenomenon may be observed much more accurately, and therefore the distance may be calculated with greater exactness.