7o6 THE POPULAR SCIENCE MONTHLY. 



in middle latitudes, in short, of all tbat makes the rivalry of nations, 

 and civilization a necessity. To answer this question it will be neces- 

 sary to turn again to astronomy, and to study for a few moments 

 some of its more abstruse problems. 



In addition to the rotation of the earth on its axis once every day, 

 and ils revolution about the sun once in a year, there is also a slow, 

 rolling motion of the equator, caused by the attraction of the sun on 

 the excess of matter in equatorial diameters over the polar. It is pre- 

 cisely as when one touches the rim of a top in rapid motion : there is 

 set up at once a slow, gyrating or tilting roll, and the upper end of the 

 stem describes a small circle. Just so the sun lays hold of the protu- 

 berant rim of the great terrestrial top, and immediately it begins to 

 oscillate in the long secular period of 25,868 years; while the polar 

 axis, extended to the heavens, describes in the same length of time a 

 small circle of 23|- radius among the northern or southern stars. This 

 is the motion which occasions what is called the precession of the 

 equinoxes. The plane of the earth's equator crosses the plane of its 

 orbit ; and, when the earth is at the points of junction, the days and 

 nights are equal the world over. These two points, therefore, are the 

 equinoxes ; and the earth passes through them about the 2tst days of 

 March and September. Owing to the rolling motion of the equator, 

 above described, these points, always in the line of intersection of the 

 two planes, pass successively through the twelve signs or constella- 

 tions, making slowly the entire circuit of the heavens. The vernal 

 equinox, which now points to, or is on a line between, the sun and the 

 constellation of the Fish, after about 26,000 years will have traveled 

 the great circle of the heavens and come back again to point to the 

 same cluster of stars which is now overhead at midnight on the 21st 

 of March. 



But the time of this revolution, so far as it affects the climate of 

 the earth, is modified by the following circumstance : The ellipse or 

 oblong circle in which the earth revolves about the sun is itself all the 

 time slowly revolving. The long diameter of it the major axis 

 makes a complete revolution in the heavens once in 110,000 years. 

 Now, as this revolution is forward, or in the same direction among the 

 constellations that the sun appears to move, while that of the equinoxes 

 is retrograde, it follows that the extremities of the major axis, which 

 are the perigee and the apogee, advance to meet the equinoctial points ; 

 so that the revolutions, or rather the conjunctions, of the equinoxes, 

 which have to do with terrestrial climate, are accomplished in the 

 shorter period of 21,000 years. 



Now, all this astronomy amounts simply to this : that in the yeai 

 of our Lord 1248 the earth was at its nearest approach to the sun on 

 the 21st day of December, our winter solstice; and that in 10,500 

 years from that time the same thing will happen on the 21st day of 

 July, our summer solstice. In the period comprising the first case, 



