A HISTORY OF SCIENCE 



and receive as accurate what they have advanced. We 

 shall also assume that the earth is spheroidal, that its 

 surface is likewise spheroidal and, above all, that bodies 

 have a tendency towards its centre, which latter point 

 is clear to the perception of the most average under- 

 standing. However, we may show summarily that 

 the earth is spheroidal, from the consideration that all 

 things, however distant, tend to its centre, and that 

 every body is attracted towards its centre by gravity. 

 This is more distinctly proved from observations of 

 the sea and sky, for here the evidence of the senses and 

 common observation is alone requisite. The con- 

 vexity of the sea is a further proof of this to those who 

 have sailed, for they cannot perceive lights at a dis- 

 tance when placed at the same level as their eyes, and 

 if raised on high they at once become perceptible to 

 vision though at the same time farther removed. So 

 when the eye is raised it sees what before was utterly 

 imperceptible. Homer speaks of this when he says : 



" ' Lifted up on the vast wave he quickly beheld afar.' 



Sailors as they approach their destination behold the 

 shore continually raising itself to their view, and ob- 

 jects which had at first seemed low begin to lift them- 

 selves. Our gnomons, also, are, among other things, 

 evidence of the revolution of the heavenly bodies, and 

 common-sense at once shows us that if the depth of the 

 earth were infinite such a revolution could not take 

 place." l 



Elsewhere Strabo criticises Eratosthenes for having 

 entered into a long discussion as to the form of the 



