ASTRONOMY. 237 



towards the end of July. But our experience tells us that the heat is 

 generally greatest in the latter part of the summer. This may be 

 attributed to the following cause. When any part of the earth's surfacfc 

 has been fairly heated, it will retain the heat for some time; and as the 

 nights, which in the middle of summer are very short, increase but 

 slowly, and the days are proportionately long, the heat of the earth must 

 be continually augmented in the day-time more than it is diminished in 

 the night, till the sun has declined considerably from the tropic, in its 

 return to the equinoctial. 



For this reason, the month of July and the greater part of August 

 will be hotter than June, unless during the former months a great deal 

 of rain should fall; in the same manner we find it hotter in the after- 

 noon than in the forenoon. By the contrary rule, places which have 

 been thoroughly cooled, will require some time to recover their heat; 

 thus the weather will be colder a month or two after the winter solstice 

 than before it. 



It is to be observed, that the sun's rays are so much refracted towards 

 the earth by the atmosphere at its rising and setting, as to bring it in 

 sight every clear day before it reaches the horizon, and keep it in view 

 for some minutes after it has descended below it in the evening. 

 Trvihylii is thus caused ; when the sun has withdrawn from our sight 

 it still continues to shine upon the atmosphere above us, from which the 

 light is reflected upon us, till it is eighteen degrees below the horizon, 

 after which, the part of the atmosphere above us, which is dense enough 

 to reflect its rays, loses them entirely, and it becomes dark. 



MERCURY 



Is the nearest planet to the sun, and performs his annual revolution in 

 eighty-eight days of our time. He is supposed to be thirty-seven 

 millions of miles distant from the sun, and that his own diameter is three 

 thousand miles ; he is computed to move at the rate of one hundred and 

 nine thousand miles an hour. His proximity to the sun prevents the 

 Astronomer from making accurate observations ; but he is supposed to 

 be peopled, although his light and heat are seven times greater than 

 those of our own earth; at his greatest distance, he is only twenty-seven 

 and a half degrees from the sun, and at other times is so near as to rise 

 and set at almost the same moment. 



