76 THE WORLD MACHINE 



indicated at a gives the elevation of the pole-star above the 

 horizon at that point. 



With a party of geometers, here quite literally " earth- 

 measurers," we take boat and cross the Mediterranean, and from 

 the Mosque of St. Sophia make the same observation. But 

 the angle is no longer the same ; the pole-star is higher up 

 toward the zenith ; the angle it makes with the plane of the 

 horizon is greater. As we have calculated the two angles in 

 degrees, then we may subtract the greater from the less ; we 

 do so, and find that the difference is about 11. Instructed 

 by Bion, we know that this change is due to the fact that the 

 pole-star is fixed, and that we have been travelling over the 

 curving surface of a globe. A moment's reflection makes it clear 



that these 11 are in reality 11 of a great circle of the earth 

 supposing, of course, that the Christian capital of Constantino 

 is directly north of Cairo (it is a little out). We have then 

 only to know the distance we have come to calculate the 

 entire circumference of the circle. 



This is to all intents the method now employed, and it 

 requires for its successful application only a water-level, an 

 angle measurer, and a known distance along a meridian. The 

 device is practically the same as that of Poseidonius, friend 

 and master of the accuser of Catiline. Instead of the pole- 

 star Poseidonius chose Canopus, which, viewed from the island 

 of Rhodes, just grazed the horizon ; at Alexandria it rose a 

 certain height above, and he measured the angle, some seven 

 degrees. 



Yet these various measures differed greatly. That reported 



