EXPLANATION. IX 



as, in common language, we associate the signs with the months in a re- 

 verse order, they are reversed) ^should really be where <YI is, aud the 

 others follow in regular order. 



The small circles round the globes represent the orbit of the moon with 

 regard to the earth, with the moon in each at the beginning of every 

 quarter. Half the moon is shewn by the little circle, and half of that is 

 left unshaded to shew the portion illuminated by the sun. At the new 

 moons, or points nearest the sun, that is wholly turned from the earth. 

 Seen from the earth, the half appears at the end of the first and third 

 quarters, the round side being toward the right hand at the first, and the 

 left at the third. At the end of the second quarter, or full moon, the whole 

 of the enlightened part is turned to the earth. At one view, however, only 

 half the moon can be represented, and therefore as much must be ima- 

 gined on the under side as is seen on the upper. A lunar eclipse, by the 

 moon's passing through the earth's shadow, is seen in the figure toward 

 the left hand ; and a solar eclipse by the moon coming between a part of 

 the earth and the sun, in the figure toward the right. In the other points, 

 the moon is supposed to be so far above or below the plane of the orbit, 

 that the shadows do not interpose. The shadows are represented as being 

 parallel, which from the magnitude and distance of the sun, is nearly their 

 position. Owing to the reflection of light by the atmosphere, the shadows, 

 even where the eclipses are total, are not absolutely dark. 



The motions of those bodies cannot be accurately represented by figures, 

 even if confined to two of them, as then it is not round the centre of one of 

 the bodies, but round the centre of gravity of the two, and that varies 

 with their distances from each other. The centre of gravity of any num- 

 ber of bodies is that point, through which if a plane or level section were 

 supposed to pass, in any or every direction, it would always divide the 

 quantity of matter into two equal parts. 



The remaining six figures on the plate are intended to illustrate the 

 variations of tide in the ocean and atmosphere, as depending on the posi- 

 tions of the sun and the moon. They are sections, in which the inside 

 circle represents the solid nucleus of the earth, the shaded oval, the tide 

 of the sea, and dotted oval that of the atmosphere. As the general prin- 

 ciple can only be shown in a figure, the solid earth is represented as 

 covered uniformly with water, and no regard is paid to the proportions. 



In those figures the horizontal lines are the axis of the earth, and the 

 vertical ones the equator, tropics, and polar circles; the lines of low water 

 have the name at length. They coincide with the axis when both lumina- 

 ries are over the equator, in every other case they form an angle with the 



