A HISTORY OF SCIENCE 



observer at the point A will continue to receive the 

 reflected rays of the sun until that body reaches the 

 point S, which is, according to the hypothesis, nineteen 

 degrees below the horizon line of the observer at A. 

 This horizon line, being represented by AH, and the 

 sun's ray by SM, the angle HMS is an angle of nineteen 

 degrees. The complementary angle SMA is, obviously, 

 an angle of (180 19) one hundred and sixty-one 

 degrees. But since M is the reflecting surface and 

 the angle of incidence equals the angle of reflection, the 

 angle AMC is an angle of one-half of one hundred and 

 sixty-one degrees, or eighty degrees and thirty minutes. 

 Now this angle AMC, being known, the right-angled 

 triangle MAC is easily resolved, since the side AC of 

 that triangle, being the radius of the earth, is a known 

 dimension. Resolution of this triangle gives us the 

 length of the hypotenuse MC, and the difference be- 

 tween this and the radius (AC), or CD, is obviously 

 the height of the atmosphere (h), which was the 

 measurement desired. According to the calculation 

 of Alhazen, this h, or the height of the atmosphere, 

 represents from twenty to thirty miles. The modern 

 computation extends this to about fifty miles. But, 

 considering the various ambiguities that necessarily 

 attended the experiment, the result was a remarkably 

 close approximation to the truth. 



Turning from physics to chemistry, we find as per- 

 haps the greatest Arabian name that of Geber, who 

 taught in the College of Seville in the first half of the 

 eighth century. The most important researches of 

 this really remarkable experimenter had to do with 

 the acids. The ancient world had had no knowledge of 



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