VARYING DECLINATION. 29 



from those points, and that for a considerable space 

 at the middle between the crossings, the two hoops 

 will be nearly parallel to each other. One of the 

 hoops in this arrangement will nearly represent the 

 direction of the motion of the whole mass of the 

 earth, in its annual progress round the sun in its 

 orbit; and the other the direction of the motion of 

 the surface, produced by the daily revolution. The 

 points where the hoops cross will represent the two 

 equinoxes, and the positions where the tioops are pa- 

 rallel, will each represent a portion of the middle 

 of summer in the one hemisphere, and the middle 

 of winter in the other. The circumstance of the 

 hoops already mentioned, will point out that the 

 angular difference of the directions of those two mo- 

 tions must both be greatest and vary most near the 

 equinoxes, and that for some time in the middle 

 of the summers and the winters, the angular differ- 

 ence must be small, and subject to little variation. 

 Those results follow as necessarily from the axis of 

 the earth being at all times in parallel positions, 

 and forming the same acute angle with the plane 

 of the orbit, as do the different portions of il- 

 lumination, or length of the day; the position of 

 the axis could not, in fact, be what it is without 

 their both following as necessary consequences. Yet 

 we shall see that this difference of angle and angu- 

 lar changes, has some influence upon the phenomena 

 of the year, which are not unworthy of our notice. 



Upon the solid mass of the earth, it has not, 

 indeed, much direct influence; neither has any ce- 

 D 2 



