164 A SHORT HISTORY OF SCIENCE 



' Choose a place in a level desert and determine its latitude. Then 

 draw the meridian line and travel along it towards the pole-star. 

 Measure the distance in yards. Then measure the latitude of the 

 second place. Subtract the latitude of the first and divide the differ- 

 ence into the distance of the places in parasangs. The result multi- 

 plied by 360 gives the circumference of the earth in parasangs.' 



Wiedemann. 



The writer just quoted describes a second method involving 

 the measurement of the angle of depression of the horizon as 

 seen from the top of a high mountain. 



It is not improbable that western Europe acquired from 

 eastern Asia, through Arab channels, the mariner's compass and 

 gunpowder. 



' When the night is so dark that the captains can perceive no star to 

 orient themselves, they fill a vessel with water and place it in the in- 

 terior of the ship, protected from wind ; then they take a needle and 

 stick it into a straw, forming a cross. They throw this upon the water 

 in the vessel mentioned and let it swim on the surface. Hereupon they 

 take a magnet, put it near the surface of the water, and turn their 

 hands. The needle turns upon the water; then they draw their 

 hands suddenly and rapidly back, whereupon the needle points in 

 two directions, namely north and south.' 1232 A. D. 



Wiedemann. 



The astronomical theory of the Arabs was merely that of 

 Ptolemy. But they "were not content to consider the Ptolemaic 

 .system merely as a geometrical aid to computation ; they required 

 a real and physically true system of the world, and had therefore 

 to assume solid crystal spheres after the manner of Aristotle." 

 The various attempts to devise a better system all miscarried, 

 their authors having no new guiding principle, nor superior mathe- 

 matical power, and being more or less hampered by Aristotelian 

 traditions, though Greek theories of the rotation of the earth 

 seem not to have been unknown. 



ASIATIC OBSERVATORIES. Besides the work of the Arabian 

 astronomers themselves, it is an interesting fact that their bar- 

 barian Mongol conquerors in the East acquired a temporarily 



