ASTRONOMICAL INSTRUMENTS. 107 



The Armillary Sphere. (Figs. 120 and 121, page 109.) 

 Of the instruments contrived to elucidate the elements of 

 astronomy, none imprint so clearly on the mind the nature 

 and use of those circles which astronomers have supposed 

 to be applied to the concave sphere of the heavens, as the 

 armillary sphere. 



If the circumference of a circle be turned about its diame- 

 ter as an axis, it will generate in its motion the surface of a 

 globe or sphere. 



And the centre of the circle will be the centre of the globe. 



All straight lines reaching from the centre of a globe to 

 its surface are equal. 



Every straight line that goes through the centre of a 

 sphere, and is terminated at both ends by the surface, is a 

 diameter. 



The diameter about which any sphere turns is its axis. 



The extremities of such diameter or axis are its poles. 



On the surface of a globe or sphere several circles may 

 be described ; those circles, whose centre is the same with 

 the centre of the globe, are called by way of distinction 

 great circles ; these divide the sphere into two equal parts. 

 The angular distance of two points situated on the surface 

 of the sphere, is measured by the arc of a great circle 

 intercepted between them. 



Lesser circles divide the sphere into two unequal parts. 



The sphere before you, by its real circles, serves to re- 

 present, and will enable me to explain to you those imagina- 

 ry circles by which astronomers divide the heavens into the 

 same parts or portions as you see these circles divide the 

 sphere. If your eye could be placed in the centre of this 

 sphere, you would see its circles upon or against those very 

 points of the heavens where the imaginary circles of the 

 astronomers are supposed to be situate. It is called armil- 

 lary, because it consists of a number of rings of brass, called 

 by the Latins armillse, from their resembling bracelets or 

 rings for the arms. 



There are six great circles of the sphere : the horizon, 

 the meridian, the equator, the ecliptic, the equinoctial colure, 

 and the solstitial colure. 



The sphere is sustained in a frame, on the top of which 

 is a broad circle representing the horizon, which represents 

 that imaginary circle which bounds or terminates the view 

 of the spectator, dividing the sphere into two equal parts ; 



