50 



THE WORLD. 



line, suppose the star to be upon it. As the earth turns on its axis, 

 the star is left behind, and after a complete revolution, the meridian 

 again arrives to it, this interval is called asiderial day, or day as 

 determined by the stars, and to ascertain this day, or its length, 

 we must have some means of determining with the utmost 

 exactness when the star is on the meridian. This is accom- 

 plished by means of the transit instrument, invented by Huygena, 

 and shown in the engraving below. 



The ordinary transit instrument consists of a telescope, A B, of 

 any convenient length, fixed firmly at right, angles to a conical 

 hollow axis, E F, the exti'emities of this axis are truly turned, 

 and rest in two angular bearings which are called Y's, since they 

 are not unlike this letter, the instrument can be lifted out of these 

 bearings, and reversed, so that the ends E and F may change 

 places. The end of the axis F, is furnished with a small graduated 

 circle C, for the purpose of reading the elevation, or altitude of 

 the body observed, and at D, is a small lamp, the light of which 

 shining into the hollow arm E, is reflected by a reflector inside 

 the tube, down to the eye. The object of this illumination is 

 to make a system of fine lines, usually raw silk, or spiders-web, 

 visible at night, at the same time with the star. In looking into 

 the transit telescope, five of these lines are usually seen, shown in 

 the engraving. A B is, by means wo cannot now describe, located 



