22 THE WORLD. 



the plane of which passes directly through the centre of the earth; 

 this is a great circle. So is A E for the same reason, for if the 

 globe were to be divided through these circles it would be exactly 

 halved, but a circle passing through H B, orF D, is called a small 

 circle, since the plane of the circle does not pass through the centre 

 of the sphere on which the circle is drawn. From this definition 

 it will be perceived that the circle A I E K, (the part behind the 

 sphere being shown by the dotted line) is a great circle, because 

 the plane of this circle passes through the centre of the sphere. 

 Every great circle, has what is called a pole, that is, a point ninety 

 degrees, or one quarter of a circle, distant from it in every direc- 

 tion, thus A is the pole of the circle G C, for from whatever 

 point on the circle G C, the distance is measured up to A, it will 

 be found 90. For instance the arcs' A G, A I, A O, A K, A C, 

 are all of their respective circles. Now suppose the circle G 

 C, to represent the equator, then A will be the north pole of the 

 earth, and E the south pole. Suppose now this great circle which 

 we have called the equator to be actually traced around the earth 

 and divided into 360 parts called degrees, marked (), and sup- 

 pose these degrees subdivided into minutes marked ('), and call 

 these minutes miles, how many miles would the earth be in cir- 

 cumference? Evidently sixty times 360, or 21,600 miles. This is 

 not so much as the circumference is usually stated to be ; viz, 

 24,000 miles, and for this reason ; the mile at the equator, is longer 

 than the English statute mile. Referring to the preceding figure, 

 it will be readily perceived that if the circle fl B was divided 

 into 360 parts and these again subdivided into 60 parts each, 

 called miles, these miles would be much smaller than the equa- 

 torial miles, indeed it would require 69| English statute miles to 

 constitute 1, or 60 equatorial, or geographical miles. Now if 

 we take 69 \ miles for the length of a degree, it is evident the 

 circumference of the earth will be 360 times this, or 25,020 miles, 

 and as the diameter is a little less than the circumference, the 

 diameter is called in round numbers 8000 miles. When there- 

 fore we assert that the earth is 8000 miles in diameter, we mean 

 simply this, if the equator, or any great circle drawn upon the 



