68 GALILEO AND HIS JUDGES. 



mathematics as well as of mechanics and astronomy. 

 Salviati is made to say that the circumference of 

 an infinite circle is identical with a straight line : 

 " sono 1' istessa cosa." This idea, familiar though 

 it be to modern mathematicians, is one that we 

 should not have expected to find enunciated in 

 the early part of the seventeenth century ; even 

 the intelligent Sagredo cannot understand or believe 

 it, and it is not further discussed ; but the fact 

 of its being here stated is especially noteworthy.* 



Another (less felicitous) guess is hazarded by the 

 same interlocutor Salviati, who, as I have already 

 remarked, appears to be the one that most nearly 

 represents the author's own mind, to account for 

 the Earth keeping her axis pointed (approximately, 

 that is to say) in the same direction during each 

 annual revolution round the Sun. Salviati suggests 

 that it may be due to some magnetic influence, 

 and that the interior of the Earth may be a vast 

 loadstone. This is strange, because it is evident 

 from what immediately preceded, that the author 

 was aware of the true reason, which in fact he 

 illustrates by the well-known experiment of a light 

 ball floating in a bucket of water, to which a revolv- 

 ing motion is imparted. It seems, however, that 

 a work by William Gilbert on the subject of 



* To speak of the circumference of a circle of infinite 

 radius as being identical with a straight line (though practically 

 true enough) is not rigidly accurate. We should say that they 

 approximate infinitely to one another, or in mathematical phrase- 

 ology, they are equal to each other in the limit. 



