THE EARTH. 



nearly the same as that to which Dr. Maskelyne had previously arrived by a 

 different method. It was thus finally established that the weight of the globe 

 of the earth is about five an<? a half times greater than the weight of its own 

 bulk of water. 



It-follows from this that the mean density of the earth is five and a half times 



greater than the density of water. We are, however, carefully to remember 



that this conclusion affects the mean density of the earth only. Now, as the 



,' density immediately at its surface is not nearly so great as this, it follows that 



'I the density of those parts nearer to its centre must be much greater. 



THE SEASONS 



' 



The succession of spring, summer, autumn, and winter, and the variations 

 of temperature of the seasons so far as these variations depend on the posi- 

 tion of the sun will now require to be explained. 



The influence of the sun in heating a portion of the earth's surface, will de- 

 pend partly on its altitude above the horizon. The greater that altitude is, the 

 more perpendicular the rays will fall, and the greater will be their calorific 

 effect 



To explain this, let us suppose ABC and D, fig. 10, to represent a beam of 



Fig. 10. 

 F D -! 



E 



- 







B 



the solar light ; let C D represent a portion of the earth's surface, upon which 

 the beam would fall perpendicularly ; and let C E represent that portion on ivhich 

 it would fall obliquely ; the same number of rays will strike the surfaces C D 

 and G E ; but the surface C E being obviously greater than C D, the rays will 

 necessarily fall more densely on the latter: and as the heating power must be 

 in proportion to the density of the rays, it follows that C D will be heated more 

 than C E in just the same proportion as C E is greater than C D. But if we 

 would compare two surfaces on neither of which the sun's rays fall perpendic- 

 ularly, let us take C E and C F. They fall on C E with more obliquity than 

 on C F ; but C E is evidently greater than C F, and therefore the rays being 

 diffused over a larger surface, are less dense, and therefore less effective in 

 heating. 



The calorific effect, of the sun's rays on a surface more oblique to their di- 

 rection than another, will then be proportionably less. 



If the sun be in the zenith, its rays will strike the surface perpendicularly, 

 and the heating effect will therefore be greater than when the sun is in any 

 other position. 



The greater the altitude to which the sun rises, the less obliquely will be 

 the direction in which its rays will strike the surface at noon, and the more 

 effective will be their heating power. So far, then, as the heating power de- 

 pends on the altitude of the sun, it will be increased with every increase of its \ 

 meridian altitude. 



