98 PHYSICAL GEOGRAPHY. 



Meridian of any place is distant from the first Meridian, 

 is said to be the Longitude of that place ; and is either 

 East or West, according to its direction, and may amount 

 to 180 degrees, the greatest Longitude that any place 

 can have. 



The being able to note the exact situation of any 

 place, is of the greatest use, in a general point of view j 

 but to the mariner who is excluded from the sight of 

 every thing but sea and sky, and can have no local mark 

 by which he may be directed to avoid rocks and whirl- 

 pools, &c., its utility is incalculable. 



The Latitude of any place may be ascertained in 

 various ways, but the most simple is from the sun's 

 meridian altitude, or height above the horizon at noon. 

 At the Equator, from which circle the Latitude is 

 reckoned, the Poles are in the horizon, while the Celestial 

 Equator passes through the Zenith ; but in proceeding 

 either northward or southward, the Pole will become 

 proportionately elevated, which elevation will be equal 

 to the Latitude of the place, or to the complement of the 

 altitude of the Celestial Equator, or Equinoctial. To find 

 the degree of Latitude, it is only necessary to ascertain 

 the height of the Celestial Equator, and to subtract 

 it from ninety degrees, the remainder will be the 

 Latitude.* 



To ascertain the Longitude mathematically, is 

 attended with considerable difficulty, and requires much 

 astronomical observation and laborious calculation. 

 The most easy and mechanical mode is by the chrono- 

 meter, a time-piece of a particularly correct nature, and 

 so constructed as not be affected by a change of tem- 

 perature. As the Earth performs a rotation on its 

 axis in twenty-four hours ; and as the Equator, or any 

 circle of Latitude, is supposed to be divided into 360 



* By adding the Sun's declination to the degree of altitude when of 

 afc'Arename, and subtracting it when of an uw//<?,name, the complement of 

 the Latitude will be obtained ; which must be subtracted from 90 

 degrees, to give the Latitude ; and this will be always of a different 

 name or kind to the altitude. Should the sum of the Sun's declina- 

 tion and altitude exceed 90 degrees, 90 must be subtracted from it, the 

 remainder will be the Latitude, and will be of the same name as the 

 declination and altitude. 



