By long and careful observations upon these satellites, astronomers have been able to construct tables, showing the exact time when each immersion and emersion will take place, at Greenwich Observatory, near London. Now suppose the tables fixed the time for a certain satellite to be eclipsed at 12 o'clock at Greenwich, but we find it to occur at 9 o'clock, for instance, by our focal time: this would show that our time was three hours behind the time at Greenwich; or, in other words, that we were three hours, or 45°, west of Greenwich. If our time was ahead of Greenwich time, it would show that we were east of that meridian, to the amount of 150 for every hour of variation. But this method of finding the longitude is less used than the "lunar method" (Art. 245), on account of the greater difficulty of making the necessary observations.

270. By observations upon the eclipses of Jupiter's moons, as compared with the tables fixing the time of their occurrence, it was discovered that light had a progressive motion, at the rate of about 200,000 miles per second.

1. This discovery may be illustrated by again referring to the opposite cut. In the year 1675, it was observed by Roemer, a Danish astronomer, that when the earth was nearest to Jupiter, as at E, the eclipses of his satellites took place 8 minutes 13 seconds sooner than the mean time of the tables; but when the earth was farthest from Jupiter, as at F, the eclipses took place 8 minutes and 13 seconds later than the tables predicted the entire difference being 16 minutes and 26 seconds. This difference of time he ascribed to the progressive notion of light, which he concluded required 16 minutes and 26 seconds to cross the earth's orbit from E to F.

2. This progress may be demonstrated as follows:-16m. 26s. 986s. If the radius of the earth's orbit be 95 millions of miles, the diameter must be twice that, or 190 millions. Divide 190,000,000 miles by 986 seconds, and we have 192,69737 miles as the progress of light in each second. At this rate, light would pass nearly eight times around the globe at every tick of the clock, or nearly 500 times every minute!

SATURN.

SATELLITES OF SATURN.

271. The moons of Saturn are eight in number, and are seen only with telescopes of considerable power. The best time for observing them is when the plant is at his equinoxes, and his rings are nearly invisible.

In January, 1849, the author saw five

of these satellites, as represented in the adjoining cut. The rings appeared only as a line of light, extending each way from the planet, and the satellites were in the direction of the line, at different distances, as here represented.

272. These satellites all revolve eastward with the rings of the planet, in orbits nearly circular, and, with the exception of the eighth, in the plane of the rings. Their mean distances, respectively, from the planet's cen

270. What discovery by observing these eclipses? (Illustrate method. Diagram. Demonstration.)

271. Number of Saturn's moons? How seen? Best time? Shape of orbits? How situated?

272. How revolve?

Periods?

Distances?

ter are from 123,000 to 2,366,000 miles; and their periods from 22 hours to 79 days, according to their dis

tances.

The distances and periods of the satellites of Saturn are as follows:

273. The most distant of these satellites is the largest, supposed to be about the size of Mars; and the remainder grow smaller as they are nearer the primary. They are seldom eclipsed, on account of the great inclination of their orbits to the ecliptic, except twice in thirty years, when the rings are edgewise toward the sun. The eighth satellite, which has been studied more than all the rest, is known to revolve once upon its axis during every periodic revolution; from which it is inferred that they all revolve on their respective axes in the same manner. 1. Let the line A B represent the plane of the planet's orbit, CD his axis, and EF the plane of his rings. The satellites being in the plane of the rings, will revolve around the shadow of the primary, instead of passing through it, and being eclipsed.

2. At the time of his equinoxes, however, when the rings are turned toward the sun (see A and E, cut, page 92), they must be in the center of the shadow on the opposite side; and the

A

SYSTEM OF SATURN-NO ECLIPSES.

F

moons, revolving in the plane of the rings, must pass through the shadow at every revolution. The eighth, however, may sometimes escape, on account of his departure from the plane of the rings, as shown in the cut.

URANUS.

274. Uranus is supposed to be attended by six secondaries. Sir Wm. Herschel recorded that he saw this number, and computed their periods and distances; and on his authority the opinion is generally received, though

273. Size? Eclipses of? When? Why not oftener? (Illustrate.) 274. Satellites of Uranus? Upon what authority? Distances? Periods! Situation of orbits? Form? Direction in revolution? Remark of Dr. Herschel?

no other observer has ever been able to discover more than three. They are situated at various distances, and revolve in from 1 day and 21 hours to 117 days. Their orbits are nearly perpendicular to the ecliptic, and they revolve backward, or from east to west, contrary to all the other motions of our planetary system. Their or bits are nearly circular, and they are described by Dr. Herschel as "the most difficult objects to obtain a sight of, of any in our system."

The distances and periods of the system of Uranus, as laid down by Dr. Herschel, are as follows:

275. Neptune is known to be attended by one satellite, and suspected of having two. Professor Bond, of Cambridge, Mass., states that he has at times been quite confident of seeing a second. The mean distance of the known satellite from its primary is 230,000 miles, or near the distance of our moon. Its period is only 5 days and

21 hours.

We have here another illustration of the great law of planetary motion explained at 74. So great is the attractive power of Neptune, that to keep a satellite, at the distance of our moon, from falling to his surface, it must revolve some five times as swiftly as she revolves around the earth. The centripetal and centrifugal forces must be balanced in all cases, as the laws of gravitation and planetary motion, discovered by Newton and Kepler, extend to and prevail among all the secondaries.

CHAPTER VIII.

NATURE AND CAUSE OF TIDES.

276. TIDES are the alternate rising and falling of the waters of the ocean, at regular intervals. Flood tide is when the waters are rising; and ebb tide, when they are

275. What said of Neptune's secondaries? Remark of Prof. Bond? Distance and period of the known satellite? (Remark in note.)

276. What are tides? Flood and ebb tides? High and low? How often do they ebb and flow?

falling. The highest and lowest points to which they go are called, respectively, high and low tides. The tides ebb and flow twice every twenty-four hours—i. e., we have two flood and two ebb tides in that time.

277. The tides are not uniform, either as to time or amount. They occur about 50 minutes later every day (as we shall explain hereafter), and sometimes rise much higher and sink much lower than the average. These extraordinary high and low tides are called, respectively, spring and neap tides.

278. The cause of the tides is the attraction of the sun and moon upon the waters of the ocean. But for this foreign influence, as we may call it, the waters having found their proper level, would cease to heave and swell, as they now do, from ocean to ocean, and would remain calm and undisturbed, save by its own inhabitants and the winds of heaven, from age to age.

In this figure, the earth is represented as surrounded by water, in a state of rest or equilibrium, as it would be were it not acted upon by the sun and moon.

NO TIDE

279. To most minds, it would seem that the natural effect of the moon's attraction would be to produce a single tide-wave on the side of the earth toward the moon. It is easy, therefore, for students to conceive how the moon can produce one flood and one ebb tide in twenty-four hours.

1. In this cut, the moon is shown at a distance above the earth, and attracting the waters of the ocean, so as to produce a high tide at A. But as the moon makes her apparent westward revolution around the earth but once a day, the simple raising of a flood tide on the side of the earth toward the moon, would give us but one flood and one ebb tide in twenty-four hours; whereas it is known that we have two of each.

ONE TIDE-WAVE.

B

2. "The tides," says Dr. Herschel, "are a subject on which many persons find a strange difficulty of conception.. That the moon, by her attraction, should heap up the waters of the ocean under her, seems to many persons very natural. That the same cause should, at the same time, heap them up on the opposite side of the earth (viz., at B in the figure), seems to many palpably absurd. Yet nothing is more true."

280. Instead of a single tide-wave upon the waters of

277. Are the tides uniform? What variation of time? As to amount? What are these extraordinary high and low tides called?

278. The cause of tides? How but for this influence?

279. What most obvious effect of the moon's attraction? (Substance of note 1 Remark of Dr. Herschel ?)

the globe, directly under the moon, it is found that on the side of the earth directly opposite there is another, high tide; and that half way between these two high tides are two low tides. These four tides,

viz., two high and two low, traverse the ocean from east to west every day, which accounts for both a flood and an ebb tide every twelve hours.

In this cut, we have a representation of the tide-waves as they actually exist, except that their hight, as compared with the magnitude of the earth, is vastly too great. It is designedly exaggerated, the better to illustrate the principle under consideration. While the moon at A attracts the waters of the ocean, and produces a high tide at B, we see another high tide at C on the opposite side of the globe. At the same time it is low tide at D and E.

TWO TIDE-WAVES

D

B

E

281. The principal cause of the tide-wave on the side of the earth opposite the moon is the difference of the moon's attraction on different sides of the earth.

If the student well understands the subject of gravitation (65), he will easily perceive how a difference of attraction, as above described, would tend to produce an elongation of the huge drop of water called the earth. The diameter of the earth amounts to about 3th of the moon's distance; so that, by the rule (69), the difference in her attraction on the side of the earth toward her, and the opposite side, would be about th. The attraction being stronger at B (in the last cut) than at the earth's center, and stronger at her center than at C, would tend to separate these three portions of the globe, giving the waters an elongated form, and producing two opposite tide-waves, as shown in the

eut.

282. A secondary cause of the tide-wave on the side of the earth opposite the moon, is the revolution of the earth around the common center of gravity between the earth and moon, thereby generating an increased. centrifugal force on that side of the earth.

The center of gravity between the earth and moon is the point where they would exactly balance each other, if connected by a rod, and poised upon a fulcrum.

Earth.

CENTER OF GRAVITY BETWEEN THE EARTH AND MOON.

Moon

This point, which, according to Ferguson, is about 6,000 miles from the earth's center, is represented at A in the above, and also in the next cut.

280. How many tide-waves are there on the globe, and how situated? 281. State the principal cause of the wave opposite the moon? (Demonstrate by diagram.)

282. What other cause operates with the one just stated to produce the tide-wave opposite the moon? (What is the center of gravity between the earth and the moon? Where is it situated? Illustrate the operation of this secondary cause. Diagram.)