50 Solar System. 



From the beginning of our era to this 

 time, it is probable, according to the best 

 accounts, that there have appeared about 

 500 comets. Before that time about 10O 

 others are recorded to have been seen, 

 but it is probable, that not above half of 

 them were comets. And, when we con- 

 sider, that many others may not have 

 been perceived, from being too near the 

 sun, from appearing in moonlight, from 

 being in the other hemisphere, from be- 

 ing too small to be perceived, or which 

 may not have been recorded, we might 

 imagine the whole number to be consid- 

 erablv greater : but it is likely, that of the 

 comets which are recorded to have been 

 seen, the same may have appeared seve- 

 ral times, and therefore the number may 

 be less than is here stated. 



Of the Moon. The moon goes round 

 the earth from change to change in 29 

 days 12 hours and 44 minutes ; and round 

 the sun with it every year. The moon's 

 diameter is 2180 miles; and her dis- 

 tance from the earth's center 24O,OOO 

 miles. She goes round her orbit in 27 

 days 7 hours 43 minutes, moving about 



