ON THE SOLAR SYSTEM. 511 



ecliptic, and sometimes as much as 7° to her own orbit. Her distance from 

 the earth is about 240 000 miles; her diameter -,?- of that of the earth, or 

 2160 miles; and the weight of bodies at her surface is supposed to be about 

 one fifth of their weiglit at the surface of the earth. 



The surface of the moon presents to us, when viewed with a telescope, a 

 great diversity of light and shade, the principal features of which are visible 

 even to the naked eye. Many of these inequalities resemble very strongly 

 the effects of volcanos; several astronomers have imagined that they have 

 seen volcanos actually burning in the unenlightened part of the planet; 

 and Dr. Herschel's Instruments have enabled him to obtain satisfactory evi- 

 dence of the truth of the conjecture. The appearance of a perforation, which 

 UUoa supposed that he observed near the margin of the Moon's disc, in a solar 

 eclipse, has been attributed by some to a volcano actually burning. Dr. 

 Halley and Mr. Weidler have also observed flashes of light on the dark part 

 of the moon, considerably resembling the effect of lightning. The height 

 of the lunar mountains has been commonly supposed to exceed very consider- 

 ably that of the mountains of the earth; but Dr. Herschel is of opinion 

 that none of them are so much as two miles high. The names, which hare 

 been given by astronomers to various parts of the moon's surface, are of some 

 utility in the observation of the progress of an eclipse. 



Of the satellites of Jupiter, some are a little larger, and others smaller than 

 the moon: they all revolve in planes inclined between Sf and 3^° to the 

 orbit of the planet, and they are therefore always seen nearly in the same 

 line. It is inferred, from some periodical changes of light which they under- 

 go, that, like our moon, they always present the same face to their primary 

 planet. > 



The ring of Saturn is inclined 31 degrees to our ecliptic; of his seven 

 satellites, six are nearly in the same plane with the ring; but the plane of 

 the seventh or outermost satellite is but half as much inclined to the ecliptic. 

 The ring has been observed by Dr. Herschel to revolve in 104- hours, which 

 is considerably less than the time that would be occupied by the revolution 

 of a satellite at the same distance. The planes of the six satellites of the 



