42 PROGRESS OF SCIENCE. 



"has been a town,'a plain, a sea, and land again." No wonder 

 he enjoyed a European reputation for hundreds of years. 



Even philosophy had, as we have seen in the case of 

 Avicenna and Averroes, abandoned the metaphysical sphere, 

 and become as positive as it well could be before the 

 XlXth century. In this respect, 



1058 mi. Algazzali is an exceedingly remarkable 

 instance of genius. He composed a work "On the CERTAINTY 

 OF KNOWLEDGE and Science," in which the deceptiveness of 

 the senses and of reason is argued with rare lucidity and 

 soundness a theory six centuries in advance of his time. 



The practical spirit of the Arabs and their passion for 

 natural and experimental philosophy are further exemplified, 

 beyond chronometry, irrigation, agriculture, ceramics, metal- 

 lurgy, and the industrial arts already noticed, by their 

 application of hydraulics in the construction of waterworks, 

 reservoirs, canals, sluices, wells, in Spain, Africa, Egypt, 

 Syria, and Mesopotamia; by their invention and intro- 

 duction into Europe of the Semaphoric Telegraph ; their 

 study of numismatics and chronology. They invented several 

 kinds of 



Clepsydras, particularly a balance clepsydra a new 

 principle altogether. They invented 



Pendulum Clocks ; they improved the 



Astrolabe ; they introduced the 



Compass ; 



Paper ; 



Gunpowder and Artillery. These three things, the com- 

 pass, paper, and gunpowder, were mighty agents of progress, 

 and brought in innumerable results in the world, in Europe 

 especially: they assisted the evolution of society in every 

 direction. Gunpowder revolutionised the social system by 

 the destruction of the all-powerful feudalism ; the compass 

 revolutionised geography, commerce, theology ; paper spread 

 ideas, and assisted in the transformation of religious and 

 scientific views. The Arabs invented the 



Aerometer; the addition of the 



Cypher to the nine earlier INDIAN NUMERALS was a 

 stroke of genius entirely due to them. There is no doubt 



