Mil EXPLANATION. 



When we speak about the celestial bodies as revolving in ellipses, we 

 must always bear in mind that this applies only to the revolving body, 

 and the centre round which it revolves, just as we speak of the nail in the 

 coach wheel revolving in a circle round the axle ; for any motion of the 

 centre will give a new form to the path of the revolving body, in absolute 

 space. We have an instance of that in the moon, which, in the course of 

 the year, will describe as many loops as there are lunations; and, as the 

 paths of the earth and moon, with regard to their respective centres 

 of revolution, are both ellipses, those loops will constantly vary in form. 



The globes upon which the meridians and parallels are drawn, represent 

 the earth, out of all proportion to the orbit of course, for as the diameter 

 of the earth is only about one twenty-five thousandth part of that of the 

 orbit, an orbit, in proportion to the earth in the figure, would have re- 

 quired a diameter of a thousand feet j and an earth in proportion to the 

 orbit, would have had only a diameter of the twelve thousandth part of an 

 inch, and would not have been visible. 



The earth in the four positions is placed with the axes parallel, but 

 making an angle of 23 with the plane of the orbit, inclined from the 

 sun at the left hand, to the sun at the rightj and inclined to the left hand, 

 but not either to the sun or from the sun, at the top and bottom. The 

 half next the sun is left unshaded to show the light, and the opposite half 

 is shaded to show the darkness at all the positions. 



The point where the meridians meet, is the north pole. The small circle 

 round it is the Arctic polar circle ; the next one the northern tropic $ and 

 the circle of which only a part is seen is the equator. Thus the figure, in 

 each of the positions, represents the same half of the earth's surface, 

 namely, the whole of the northern polar zone, the whole of the northern 

 temperate zone, three-fourths of the northern half of the torrid zone, and 

 one-fourth of the southern. 



The meridians on the globes are hours asunder j twelve mid-day being 

 always nearest the sun, the six o'clocks equally distant, and twelve mid- 

 night the most remote. The number of hours of sunshine or darkness, at 

 any latitude and any season, may be estimated by noticing how many of 

 hour lines are light and how many dark j for instance, ten hours of the 

 northern tropic are illuminated at the winter solstice, and fourteen at the 

 summer, and twelve of every latitude of the equinoxes. 



The dotted circle outside the earth's orbit, is merely a mark in space, to 

 represent the plane of the orbit as produced to the regions of the stars. 

 The globes are at the commencement of the signs for the beginning of the 

 quarters ; the other commencements are at the small lines across the 

 dotted circle ; and the usual marks are put for the signs themselves. But 



