107 



largeness and correctness of the instruments fhfiy made 

 use of, much exceeding what the m xlerns w >uld be 

 willing to believe ; the multitude of their observai'L-ns 

 and writings being six times more than wbat have been 

 composed by Greeks and Latins; and, in short, the 

 number of powerful princes, who, in a manner be- 

 coming their own magnificence, aided them with pro* 

 tectioni One letter is not sufficient," says he, u to 

 shew in how many respect* the Arabian astronomers 

 detected the deficiency of Ptolemy, and the pains they 

 took to correct aim; how carefully they meas i| red 

 time by water-clocks, sand. glasses, immense solar 

 dials, and even what perhaps will surprise you, the 

 vibrations of the pendulum ; and with what assiduity 

 and accuracy they conducted themselves in those nice 

 attempts, which do so much honour to human genins 

 in the taking the distances of the stars, and the mea* 

 sure of the earth.'* 



2. Hence it is manifest that the vibration of the 

 pendulum was employed by the ancient Arabians long 

 before the epocha we ordinarily assign for its first 

 discovery; and the use it was applied to, was exactly 

 to measure time, the very purpose for which we em- 

 ploy it. 



3. The discovery of the refraction of light, is of 

 more ancient origin than is generally imagined, for the 

 cause of it appears to have been known to Ptolemy. 



'According to Roger Bacon's account, that great phi- 

 losopher and geometrician gave the same explanation 

 of that phenomenon, which Descartes has done since : 

 for he says, t; that a ray, passing front a more rare 

 into a more dense medium, becomes more perpendicu* 

 lar." Ptolemy wrote a treatise on, optics, which was 

 ex ait in Bacon's time; and Alhazen seems not only 

 to nave known that treatise of Ptoie ny, but to have 

 drawn thence whatever is truly estimated in wn.dt he 

 advances about the refraction ot light, astronomical 

 aud the cause of the extraordinary size oi 



