time; but variation in the temperature of the air renders their movements more or less irregular.

It is not always necessary to wait for the sun to come to the meridian, in order to know the time of day. It may be known by other ways, as by the rising and setting of the sun, and of stars near the equator.

78. Hence it is often desirable to correct timepieces at sea; and for this purpose eclipses of the moon are sometimes of use. For eclipses of the moon take place at precisely the same time to all to whom the moon is visible; which is not the case with eclipses of the sun, as will be shown hereafter. Thus, if I sail from London, having an almanac in which the precise time of the beginning or ending of a lunar eclipse is calculated for the time at London; if the moon is visible to me at the time of eclipse, by observing the time of its beginning or ending, I get the true time at London, and can correct my timepiece accordingly. For example, if, on a particular day, an eclipse of the moon is calculated to begin at 17 minutes past 11 in the evening, London time, and at sea I observe it begin at 12 minutes past 11 by my chronometer, I know the chronometer is 5 minutes too slow, that is, slower than London time, and I can correct it accordingly.

Though eclipses of the moon take place at the same instant to all spectators, it is difficult to tell the precise moment when they begin or end, as will be explained in its proper place.

79. Hence if eclipses were frequent, timepieces, by being often regulated, would generally show correct time. But it is only in comparatively few voyages, during his life, that the navigator has opportunity of witnessing a lunar eclipse. On this account astronomers have turned their attention to the eclipses of other bodies, and especially to those of the satellites of JupiThese eclipses, like those of the moon, take place at the same instant to all spectators, and are suf

ter.

ficiently frequent for correcting timepieces at sea,there being scarcely a day, during which one or more of these satellites is not eclipsed. But at present it appears impossible to realize the peculiar advantages, which these phenomena are calculated to afford. For the satellites of Jupiter are too small to be visible to the naked eye, and the motion of the vessel renders a telescope useless. Hence, although astronomers have taken great pains to calculate these eclipses, yet they seem to have added nothing to the customary means of finding longitude at sea.

80. There is a method of ascertaining the time at London by observing the moon's place. Tables are calculated, showing the distance of the moon from the sun and some fixed stars for every day at noon, and every three hours afterwards, London time. To explain the use of these tables, suppose on a particular day it is stated in them that the moon will be 65° eastward from the sun at 6 o'clock in the evening, London time. At sea I observe that the moon is not 65° eastward from the sun till 40 minutes past 7, by the time where I am. This difference of 1 hour 40 minutes gives a longitude of 250; and this must be eastward, because the time where I am is later than that at London. To an astronomer, accustomed to the application of the neces sary principles, this method of finding the longitude would be the most accurate. Tables of the moon's parallax have been lately calculated; so that this method of finding longitude is accommodated to the capacity of the mass of navigators, and is daily coming more into But on account of the moon's parallax, which will be explained hereafter, it has hitherto been difficult to apply these tables.

use.

81. Notwithstanding these various methods of finding longitude, it is still very difficult. An easy, expeditious, and sure method of effecting this purpose is a great

desideratum. Such a discovery would constitute a new era in navigation, scarcely less important than that of the discovery of the mariner's compass. The English nation, to whom every facility in the improvement of commerce is particularly important, have used all suitable means to direct the attention of astronomers to this subject. By an act of parliament, passed 1714, the English government offered 20,000 pounds reward to any person who should discover a method of finding longitude at sea within 30 miles, or a degree; 15,000 pounds, if within 40 miles, or of a degree; and 10,000 pounds, if within 60 miles, or a degree. Mr. John Harrison, an eminent artist, obtained, at two different times, 20,000 pounds for improving chronometers. So exact was one of his construction, that it erred but 1 minute 54 seconds in 5 months, a mean daily error of second. By a new act of parliament, passed 1774, the greatest reward which can now be obtained is 10,000 pounds.

TABLE

Showing the length of a degree of Longitude for every degree of Latitude, in geographical miles.

CHAP. III.

82. In the short account given of the solar system in Chap. I, we attempted to describe the appearances of the various heavenly bodies, and to state such facts relating to them, as are known to exist. But there are many particular appearances and phenomena peculiar to each planet, arising from its situation in the solar system, from its revolution on its axis, from its revolution round the sun together with the degree in which its equator varies from its ecliptic, (called the obliquity of the ecliptic,) from its atmosphere, and from its size.

These phenomena are of little use or interest to us, as they affect the inhabitants of other planets; but are of great use as they affect us. Hence we shall confine ourselves to such as relate to the earth, and are of constant ob

servation.

SECT. I.

Of Phenomena arising from the situation of the Earth in the Solar System.

ART. 1. Of the different apparent motions and magnitudes of the other planets.

83. The primary planets seen from the sun always appear to move the same way, viz. from west to east, which is their direct motion. But as seen from any planet, all the rest appear to move from west to east part of the time, to be stationary part of the time, and to move from east to west part of the time; (which last is called retrograde motion. Pl. III. fig. 3.) Let S be the sun, E the earth, and a, b, c, d, e, f, g, h, Venus in different points in her orbit. It is plain, that while Venus is passing from d to f, it will appear to move in the starry heavens in the direction from o to n, whe