Elements of Astronomy. 15 



the quadratures, at the time of the equinoxes, it exceeds its 

 mean state by 7' 49", 2 ; and is in a similar manuer dimi- 

 nished by this quantity, at the time of the solstices. 



The height of the tides is also considerably influenced 

 by all those causes which have been just mentioned ; and 

 depends on the phases and posiiion of the Moon in her or- 

 bit. It is greatest when the Moon is in the syzigies ; and 

 is diminished in the quadratures. The distance likewise of 

 the Sun and Moon from the Earth, as well as their declina' 

 tion, has a material effect upon the iieight of the tides. 



But the state of the ticies is so modified by the nature 

 and position of the coasts, the depth of the channel, the 

 operation ot the winds, and by other causes, that the above 

 Jaws will not always be found to correspond with the actual 

 state of the tides, particularly near the coast, or in rivers. 

 It will however be found, from the mean of a number of 

 observations, that the inequalities in the heights and in the 

 intervals of the tides have various periods. Some are of 

 half a day and a day ; others are of half a month and a 

 month; whilst others again are of half a year and a year: 

 and some are the same as the times of the revolutions of 

 the lunar nodes and apsides. 



The action of the Moon upon the waters of the ocean is 

 triple that of the Sun. 



Mars. 



Mars is easily known in the heavens by his red and fiery 

 appearance. He performs his sidereal revolution in OSS'* 

 es** 30' 39",0 or in 1'861 Julian years: and his mean syno- 

 dical revolution in about 7&0 days, or in about 2' 133 years. 



His 7?2eG?i distance from the Sun is 1'524; that of the 

 Earth being considered as unity. This makes his mean 

 distance above 142 millions of miles. 



The eccentricity of his orbit is '093: half the major axis 

 being considered as unity. 



His mean lo?igil?ide, at the commencement of the pre- 

 sent century, was in 2' 4° 7' 2", 3. 



The longitude of his perihelion was, at the same time, in 

 11" 2° 24' 23",9 : but the line of the apsides has an appa- 

 rent motion, according to the order of the signs, of 1' 5'',9 

 in a year, or I'^ 49' 52'',4 in a century. 



His orbit is inclined to the plane of the ecliptic in aft 

 angle of l°3r3'',5: which angle decreases about i'',4 in 

 a century. 



His orbit at the commencement of the present century 

 crossed the ecliptic in r 18° l' 28",0; but the place of the 



?iodcs 



