Gtrtjbb — A Circumferentor. 145 



If desired, a magnetic compass can be fixed to the lower box ; 

 and then all his bearings can be taken with reference to the 

 magnetic meridian. 



A striking advantage of this form of instrument is that there 

 is not the same necessity for accuracy of mechanical fitting as in 

 the case of the ordinary forms of measuring instruments ; for 

 instance, a slight looseness in the vertical spindle, which is pur- 

 posely left very free in order that it may be set without the 

 slightest danger of moving the lower box, will not produce any 

 error in the readings, because a shake or loss in this part affects 

 equally the object observed and the scale projected upon it, but 

 does not affect the accuracy of the superposition of one upon the 

 other. 



Plate XI. represents the instrument in its complete form, 

 mounted upon a tripod stand. It will be seen that the whole 

 instrument can be turned with its central axis into a vertical or 

 horizontal position, so that vertical angles can be taken as well as 

 horizontal. 



