THE TRANSIT INSTRUMENT. 



admirable contrivance. They have enabled observers to mark for 

 themselves the meridian upon the firmament with such distinct- 

 ness and precision, that the moment at which any celestial object 

 passes it can be ascertained to a small fraction of a second by 

 direct observation. 



One of the forms of instrument most easily understood by which 

 'this is accomplished is shown in fig. 1 (p. 113). The passage of any 

 celestial object across the meridian being called a TRANSIT, in- 

 struments adapted to ascertain the moment such transits take 

 place are called TRANSIT INSTRUMENTS. The particular form 

 shown in fig. 1 is called a TRANSIT CIRCLE. 



The instrument is mounted on two pillars, A c and B D, of solid 

 stone, erected on a foundation of masonry presenting all the con- 

 ditions necessary to guarantee the greatest firmness and solidity. 

 These pillars stand east and west, the space between them there- 

 fore,- looking north and south. A telescope E F is supported 

 upon an horizontal axis A B, the ends of which rest in angular- 

 shaped supports, called from their form Y's, which are established 

 upon the summits of the two stone pillars. These supports being 

 rendered by suitable adjustments truly horizontal, and the line 

 joining them being directed truly east and west, the telescope 

 when placed in an horizontal direction will point exactly north 

 and south, and if it be turned upon its axis, so as to be succes- 

 sively directed to different points of the firmament, it will sweep 

 over the celestial meridian. 



Attached to the telescope is a graduated circle, consisting of two 

 flat rims of metal, connected together in a firm manner by a 

 system of spokes and diagonal braces. By means of this circle the 

 altitude of any object to which the telescope may be directed can 

 be measured ; but this not being connected with our present pur- 

 pose need not be further noticed. All that is now necessary to 

 be understood is that when it is turned upon its axis, the telescope 

 is successively directed to all points of the meridian. 



When we look through the telescope, we behold a circular space 

 upon the heavens of a certain magnitude. This space is called 



the FIELD OF VIEW. 



The meridian is in the direction of a line which would pass 

 vertically through the centre of this circular space, dividing it 

 into two equal parts, one to the right, and the other to the left. 

 The celestial objects, as they are carried by the diurnal motion of 

 the sphere, pass from east to west across the meridian, moving in 

 a direction apparently horizontal. Such of them, therefore, as 

 may come within the limits of the field of view, in any one posi- 

 tion of the telescope, will appear to pass across the field in 

 horizontal lines ; and if the observer were provided with any means 



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