SCIENCE OF COMMON THINGS. 133 



Heat of the sun. Cause of spring, summer, and winter. 



The heat of the sun passes readily through glass, 

 whereas this property is possessed by artificial heat in 

 a very small degree. 



836 What is the generally received opinion at present, in regard to the 

 actual temperature of the visible surface of the sun 1 



That the temperature of its luminous coating is much 

 more elevated than any artificial heat we are able to 

 produce. 



837 Why is the heat of the sun always greater in some portions of the 

 earth than at others ? 



Owing to the position of the earth's axis, the rays of 

 the sun always fall more directly upon the central por- 

 tion of the earth than they do at the poles or extremities. 



838 Upon what does the succession of spring, summer, autumn, and 

 winter, and the variations of temperature of the seasons, depend ? 



Chiefly upon the position of the sun in relation to the 

 earth. 



839 When do we experience the greatest amount of heat from the rays 

 of the sun f 



When its rays fall most perpendicularly. 



840 Why is the heat of the sun greatest at noon ? 



Because for the day the sun has reached the highest 

 point in the heavens, and its rays fall more perpendicu- 

 larly than at any other time. 



84:1 Why is it warmer in summer than in winter f 



Because in summer the position of the sun is such 

 that its rays fall more perpendicularly than at any other 

 season. The sun is longer above the horizon in summer 

 than in winter, and consequently imparts the greatest 

 amount of heat. 



843 Why is it colder in winter than in summer f 



Because in winter the position of the sun is such that 

 its rays fall more obliquely upon the earth than at any 

 other season. The sun is also for a less time above the 

 horizon in winter than in summer, and consequently 

 imparts less heat. 



843 Upon what does the heating power of the sun depend in a great 

 measure ? 



