532 



THE LATITUDES AND LONGITUDES. 



"\ 



If we imagine a circle surrounding the surface of the globe in such a man- 

 ner as to divide it into two hemispheres, having in the midst of one the north 

 pule, and in the midst of the other the south pole, such a circle is called the 

 cq/iutiir, and is so called from equally dividing the globe. Every point in this 

 circle will be at the same distance from the poles, and if we imagine the globe to 

 !>e cut by a plane through the poles, that plane will be at right angles to this 

 circle, and the section it forms will be what is called a terrestrial meridian. 

 The arc of this meridian between either pole and the equator will be one quar- 

 ter of its entire circumference, and will therefore be 90. The equator is, 

 therefore, everywhere 90 from each of the poles. 



The hemispheres into which the equator divides the earth are called the 

 northern and southern hemispheres. That which includes the north pole, being 

 the northern, and that which includes the south pole, the southern. 



The position of a place in either hemisphere with reference to the equator 

 is expressed by stating the number of degrees of a terrestrial meridian included 

 between the place and the equator. This is called the latitude of the place; 

 which is the distance of the place from the equator expressed in degrees of 

 the meridian. Thus, if a place be midway between the pole and the equator, 

 its latitude is 45. If it be distant from the equator by two thirds of the entire 

 distance from the equator to the pole, its latitude will be 60 and so on. 



The latitude is said to be northern and southern, according as the place is 

 in the northern or southern hemisphere. 



But it is evident that the latitude alone will be insufficient for the determina- 

 tion of the position of a place. If we state that a certain place is 45 north 

 of the equator, it will be impossible to ascertain certainly the place in question, 

 inasmuch as there is a circle of points on the earth, all of which are 45 north 

 of the equator. If we suppose a line drawn on the surface of the r.orthern 

 hemisphere parallel to the equator, at the distance from the equator of 45, 

 every point of such line or circle will be equally characterized by the latitude 

 of 45 3 north. 



Such a circle is called a parallel of latitude, and it is therefore apparent that 

 wherever such a parallel may be drawn upon the earth, all the places upon it 

 will have the same latitude. 



The latitude is, then, insufficient to determine the position of any place. 

 How, then, it may be asked, can the exact position of any place be expres- 

 sed 1 



Let us suppose that a meridian is arbitrarily selected, passing through some 

 particular place, such as the Capitol at Washington. We may conceive an- 

 other meridian drawn upon the earth east or west of that, so that the two me- 

 ridians shall include between them an arc of the equator, consisting of a defi- 

 nite number of degrees ; say, for example, that it shall consist of 20 ; then 

 such a meridian will be defined by stating that it is 20 east or west of the 

 menuim of Washington. All that can be settled by such a statement is the 

 position of the meridian in which the place lies with reference to the arbitrarily 

 chosen meridian of Washington. This relative position of the two meridians 

 is called the longitude of the place. As the meridian from which the longitude 

 is measured is altogether arbitrary, there being no physical or geographical 

 reason why one meridian should be chosen rather than another, each nation 

 has naturally selected as the zero of longitude the meridian of some noted 

 place in its precincts. In England, the Royal Observatory at Greenwich h-;.s 

 been the place selected, and accordingly in all English works on geography, 

 political and physical, longitudes are invariably expressed in reference to the 

 meridian of Greenwich. It will, therefore, be most convenient for us here 

 chiefly to refer to that meridian. 



