5O8 SCIENTIFIC RECREATIONS. 



longer, and at the north and south poles day and night are continuous, for 

 the small spaces round the poles are, during a certain period, wholly in sun- 

 shine and shade respectively. 



In March (in the diagram, fig. 546) we see that exactly one half of 

 the earth is illuminated, and the other is darkened. So in September, when 

 we have the opposite view. In June the earth is more inclined apparently 

 to the sun, and more of the surface is exposed to it, so the days are longer 

 in some parts. The opposite effect is visible in December. 



The summer heat and winter cold are accounted for by the more or 

 less direct force of the sun's rays, for the more the angle of incidence is 

 inclined the fewer rays reach the object; and if the rays fall at an angle of 

 60, the heat is only half what it would be if they came vertically. When 

 the days are shortest the sun is lowest, and therefore gives less heat to the 

 earth at certain periods. 



The wonderful precision which has adapted the position of the earth 

 on its axis, will be apparent from the illustration (fig. 557). Here we have a 



table and some bottles, a candle to repre- 

 sent the sun, and a ball of worsted and a 

 knitting-needle to represent the earth and 

 its axis. Suppose we place the ball in the 

 position at a, with its axis perpendicular 

 to the plane of the orbit. As the earth 

 would turn and go round the sun in this 

 supposed case, we should find the days and 

 nights equal, and the sun would quickly 

 scorch up the tropics, and the other por- 



557. inclination of A*;? tions would have 3. never-changing spring 



or winter all the year for ever. This would not be so pleasant, for variety 

 is the charm of nature, and the salt of life. So we may put a aside, as the 

 earth would be scarcely habitable under the supposed conditions, and try b. 

 Here we find the poles directed to the sun. The whole northern hemi- 

 sphere would thus be illuminated one half year, and the southern similarly ; 

 such rapid changes from heat to cold and back again would not suit us. So 

 we fall back upon c, the actual appearance of the position of the earth, and 

 here we find all the most favourable circumstances existing for us. This 

 inclination gives rise to all the varied phenomena of the pleasant gradations 

 of heat and cold, summer and winter, the charming changes of season, and 

 the wonderful results of the ever-recurring days and nights, months and 

 years, as the earth spins round. So we see that the sun does not really rise 

 and set upon the earth ; the globe rotates, and brings us into view of the 

 sun, and as we turn we lose his light. 



In the foregoing brief description we have learnt some few facts con- 

 cerning our earth. We have ascertained that the planet we inhabit is 

 round; we have also seen that the earth moves around the sun and around 

 its own axis, and also that it moves at a tremendous rate; we know that that 



