xlii INTRODUCTION TO ASTRONOMY. 



them ; if to the west, it is six o'clock in the morning:, and the sun will be 

 proceeding towards that meridian. 



Those circles which divide the globe into two equal parts, such as the 

 equator and the ecliptic, are called great circles to distinguish them from 

 those which divide it into two unequal parts, as the tropics and polar 

 circles, which are called small circles. All circles are divided into 360 

 equal parts, called degrees; and these degrees into 60 equal parts, called 

 minutes. The diameter of a circle is a right line drawn across it, and 

 passing through the centre ; the diameter is equal to a little less than 

 one-third of the circumference, and consequently contains a length equal 

 to nearly 120 degrees, or more accurately, about 114J degrees, of the circle 

 itself. A meridian, reaching from one pole to the other, is half a circle, 

 and therefore contains 180 degrees ; and the distance from the equator to 

 the pole is half of a meridian, or a quarter of the circumference of a circle, 

 and contains 90 degrees. 



Besides the usual division of circles into degrees, the ecliptic is divided 

 into twelve equal parts, called signs, which bear the name of the constella- 

 tions through which this circle passes in the heavens. The degrees mea- 

 sured on the meridians from north to south, or south to north, are called 

 degrees of latitude ; those measured from east to west on the equator, or 

 any of the lesser circles parallel to it, are called degrees of longitude ; 

 these lesser circles are called parallels of latitude, because being every 

 where at the same distance from the equator, the latitude of every point 

 contained in any one of them is the same. 



The degrees of longitude must necessarily vary in length according to 

 the dimensions of the circle on which they are reckoned : those, for 

 instance, at the polar circles, will be considerably smaller than those at 

 the equator. The degrees of latitude, on the contrary, never vary in 

 length, the meridians on which they are reckoned being all of the same 

 dimensions. The length of a degree of latitude is 60 geographical miles, 

 which is equal to 69 j English statute miles. The degrees of longitude 

 at the equator would be of the same dimensions were the earth a perfect 

 sphere ; but its form is not exactly spherical, being somewhat pro- 

 tuberant about the equator, and flattened towards the poles. This form 

 proceeds from the superior action of the centrifugal power at the equator. 

 The revolution of the earth on its axis gives every particle a tendency to 

 fly off from the centre. This tendency is stronger or weaker in proportion 

 to the velocity with which the particle moves. Now a particle situated 

 near one of the polar circles makes one rotation in the same space of 

 time as a particle at the equator; the latter, therefore, having a much 

 larger circle to describe, travels proportionally faster, consequently the 

 centrifugal force is much stronger at the equator than at the polar circles : 

 it gradually decreases as you leave the equator and approach the poles, 

 where, as there is no rotatory motion, it entirely ceases. Supposing, 

 therefore, the earth to have been originally in a fluid state, the particles 

 in the torrid zone would recede much farther from the centre than those in 

 the frigid zones : thus the polar regions would become flattened, and 

 those about the equator elevated. According to the same rule, our heads 

 move with greater velocity than our feet ; and on the summit of a moun- 

 tain, our velocity is greater than in a valley ; for the head is more distant 

 from the centre of motion than the feet the mountain-top than the valley. 

 Even at the equator, however, the force of gravity preponderates very 

 considerably, being at the equator 288 times greater than the centrifugal 

 force. 



