METEOROLOGY. 



323 



In obtaining these tabular results, the earth's mean distance from 

 the sun was taken at 95,273,810 miles, and its radius at 3,962 miles. 



It will be perceived that the vast magnitude of tlie sun brings ad- 

 vantages of temperature and sunlight similar to those which the pre- 

 ponderance of its mass gives to the steadiness and uniformity of the 

 planetary revolutions. Were the same amount of heat and light 

 radiated from a smaller body like the moon, the effects would be re- 

 stricted to a smaller portion of the earth's surface ; and the zone of 

 differential radiation would be reversed to one of cold and darkness. 

 But in the present beneficent arrangement, light and heat prepon- 

 derate, counteracting extremes of heat and cold with a warmer 

 temperature. And this effect is further prolonged by atmospheric 

 refraction and reflection of the rays, which, rendering the transitions 

 more mild and gradual, lessens the reign of night. 



To estimate the effect of the Refraction of Light, we have only to 

 find two points on the spherical surface of the earth, at such distance 

 that the inclination of the two tangent rays from the sun falling on 

 them shall be just equal to the horizontal refraction; that is, suppose 

 the sun's upper margin or limb to be in the horizon, sending without 

 refraction a level beam of rays to the observer. In consequence of 

 horizontal refraction in the atmosphere, the rays will appear to come 

 from a source 34' higher in altitude. And being inclined at this angle 

 with the unrefracted rays, they will pass over, and become tangent to 

 a point of the eartli's surface 34' of terrestrial arc behind the former. 

 The terrestrial radii drawn to these points will evidently be inclined 

 at the same angle as their tangents, which is 34' nearly, corresponding 

 to a distance on the surface of 40 English miles. Thus it appears 

 that the effect of refraction in widening the irradiated zone of the 

 earth is more than twice as great as that arising from the apparent 

 semi-diameter, or the mere size of the sun. Uniting the two effects, 

 the sun is found to illuminate more than half of the earth's surface 

 by a belt or zone that is 58 miles in width, encircling the seas and 

 continents of the globe. 



The advantage of the vast size of the sun is most conspicious upon 

 the planet Venus, our evening and morning star, where the belt of 

 illumination is sixty-one miles in width, as shown in the preceding 

 table. The next in rank is Jupiter, whose belt of greater illumina- 



