THE CONQUEST OF TIME AND SPACE 



diameters would maintain the horizontal position 

 through the effect of gravity. A superior quadrant of 

 the circle was marked with degrees and minutes. A 

 straight piece of metal, with sights so that it could be 

 accurately pointed, was adjusted to revolve on a pivot 

 at the center of the circle. This sighting piece being 

 aimed at the sun, for example, the elevation of that 

 body could be read directly on the measuring arc of 

 the circle. Here, then, was no new principle involved, 

 but the instrument had obvious points of advantage 

 over the cross-staff, in particular because only a single 

 sight need be taken, the horizon line being determined, 

 as already explained, through the action of gravitation. 

 The astrolabe did not gain immediate favor with 

 practical navigators, and it was at best a rather clumsy 

 instrument, subject to peculiar difficulties when used 

 on a rolling ship. Many attempts were made to im- 

 prove upon it, but for a long time none of these was al- 

 together successful. The final suggestion as to means of 

 overcoming the difficulties encountered in measuring 

 the altitude of astronomical bodies was made by Sir 

 Isaac Newton. But nothing practical came of his 

 discovery, as it was not pubHshed until a long time after 

 his death. Meantime independent discovery of th^ 

 same principle was made by Thomas Godfrey of | 

 Philadelphia, in 1730, and by the English astronomer 

 Hadley, who published his discovery before the Royal 

 Society in 1731. The instrument which Hadley de- 

 vised was called a quadrant. The principle on which it 

 worked involved nothing more complex than the use of 

 two mirrors, one of them (known as the horizon gh 



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