28 HEAT. 



Hmmuchheat 49. Q.UANTITY OF HEAT THE SUN SENDS 



does the. sun , 



send to the TO THE EARTH. The sun sends enough 

 earth? j ieat to tne eartn every year to melt a shell 



of ice enveloping the earth a hundred feet thick. This 

 may be ascertained by observing what thickness the 

 average heat of the sun will melt per minute, and then 

 calculating the quantity for a year. The method ac- 

 tually pursued is slightly different from this, but the 

 same in principle. The sun, in fact, sends a larger 

 amount of heat to the earth than is above stated, but 

 40 per cent, of it is absorbed by the atmosphere. The 

 quantity above given is the remaining 60 per cent. 



50. TOTAL QUANTITY OF HEAT THE SUN 



Howmuchheat . 



is given out by GIVES OUT. KllOWlllg flOW much COniCS 



the sun audits t th th d it atm osphere, it is easy 



atmosphere ? * 



to calculate how much starts from the sun. 

 It is just in proportion to the extent of the whole visi- 

 ble heavens, as seen from the sun, compared to the space 

 occupied by the earth, as seen from the same point. 

 By making the calculation it is ascertained that a 

 quantity of heat is given out from the sun in a year 

 which, if it all came to the earth, would melt a crust 

 of ice nearly 4000 miles thick, or a quantity which 

 would melt every minute a crust nearly thirty-seven 

 feet in thickness. But the heat of a blast-furnace, 

 if kept up constantly to the highest point, would melt 

 'but a little over the thickness of five feet of ice per min- 

 ute. The sun's surface is, therefore, more than seven 

 times as hot as the glowing surface of the fire of a 

 blast-furnace. 



