INTRODUCTION TO ASTRONOMY 



XXXV 



Fig. 6. 



TT 9 9 



Mercury is the planet nearest the sun : his orbit is consequently con- 

 tained within ours ; but his vicinity to the sun occasions his being nearly 

 lost in the brilliancy of his rays ; and when we see this planet, the sun is 

 so dazzling, that very accurate observations cannot be made upon him. 

 He performs his revolution round the sun in about 87 days, which is 

 consequently the length of his year ; the time of his rotation ou his axis 

 is not accurately known ; his distance from the sun is computed to be 

 37 millions of miles, and his diameter 3, "224 miles. The heat of this planet 

 is so great, that water cannot exist there but in a state of vapour, and 

 metals would be liquefied. 



D 2 



