60 HISTORY OF SCIENCE. 



benefited by the general and long-sustained belief in the delusions of 

 astrology; for the desire of penetrating into the future supplied a 

 powerful motive for unremitting and accurate observation at periods 

 when the love of science for its own sake might have failed to secure 

 such a result. To astrology was owing the ardour with which the 

 ancient Chaldeans scanned the heavens ; and to the same cause we 

 can trace the encouragement which many European princes, eager to 

 read the course of human affairs through the starry influences which 

 ruled them, so lavishly bestowed upon astronomers before the revival 

 of letters and subsequently. 



The list of Arabian astronomers is a long one, and a vast number 

 of the observations they have recorded are still extant to testify to the 

 accuracy and diligence of their compilers. It would be without in- 

 terest to the general reader to give here a dry record of their several 

 labours ; but some of the more remarkable must be briefly mentioned. 

 The astronomers of the Khalif El Mancoura were directed by him to 

 make a new and more exact measurement of the length of an arc of 

 the meridian. This determination was made on the plains of Meso- 

 potamia, where two parties started from a given point, one measuring 

 a degree northwards, the other southwards. The distances obtained 

 were apparently 56 miles and 561 miles respectively; but as there is 

 some doubt about the exact value of the units of length in which the 

 results are recorded, we are unable to judge confidently of their cor- 

 rectness. 



The most distinguished of all the Arabian astronomers was perhaps 

 ALBATEGNIUS, otherwise called EL-BATANI, whose observations were 

 far superior in accuracy to any recorded by the Greeks. Yet the 

 Arabs do not seem to have employed any other than instruments of 

 the same kind as those of the Greeks, viz., the gnomon, armillary 

 spheres, and mural quadrant^ El-Batani over-corrected Ptolemy's 

 estimate of the rate at which the precession of the equinoxes takes 

 place. The Greek astronomer had made it 100 years for one degree. 

 El-Batani's data gave him 66 years for one degree ; but the true period 

 is 72 years. An Egyptian astronomer named IBN TOUNIS, who about 

 the year A.D. 1000 established an observatory at Cairo, drew up a set 

 of tables containing many improvements on those of Ptolemy, and 

 left a treatise on astronomy, which is remarkable for showing greatly 

 improved methods of applying tngpjipjnetrical Jormulae. The science 

 of astronomy would have made but "small progress without trigono- 

 metry, which treats of the relations between the angles and sides of 

 triangles. By an investigation of these we are able to "resolve" 

 triangles ; that is, when any two angles and one side, or two sides and 

 an angle, or the three* sides of a triangle, are known, we are enabled 

 by trigonometry to calculate all the remaining parts of the triangle. 

 The Greeks investigated some of the purely geometrical relations of 

 angles, but they did not advance to trigonometry, because they had 



