4 GRAVITY, AND ITS EFFECTS. 



sible for the surface, the earth itself not being in 

 motion. 



The earth, however, is not at rest; it turns 

 round once every day on its axis; so that every 

 point on its surface travels round a circle. The 

 circumferences of these circles answer to the paral- 

 lels of latitude of the globe. Their diameters are 

 not equal, but increase from the poles to the 

 equator, where their length is greatest. Now you 

 are aware, that any body whirling in a circle, like 

 a weight hung by a string, is always trying to fly 

 from the centre round which it moves, and does 

 actually fly from it, if not kept back by a sufficient 

 power. This tendency is called the "centrifugal 

 force" Its strength increases, as may be ascer- 

 tained at once by the simplest experiment, in 

 proportion to the mass of the whirling body, and 

 to the quickness of its motion. 



If two bodies of equal weight accomplish their 

 revolutions in equal times, as is the case with all 

 bodies on the earth, that one gains the greatest 

 centrifugal force which travels in the greatest 

 circle. 



Wow, by reason of the daily rolling of the earth 

 round its axis, every particle of it must also get 

 this centrifugal tendency. This is greatest at the 

 equator, and, diminishing as the latitude increases, 

 vanishes at the poles. 



Under the equator, the centrifugal force is di- 



