184 Mr. William Galbraith, on some 



All the more usual astronomical instruments have a level 

 applied to them so as to insure the verticality of their axis, 

 or to make the necessary allowance for their deviation from 

 it. The scale of the level is so graduated as to show single 

 seconds, or some multiple of the second, and reads most 

 conveniently from a central zero. In those instruments 

 that revolve in azimuth, which all the smaller, and more 

 especially the portable circles do, (and even the large eight 

 feet circle at Dublin, though provided with a plumb line 

 rather inconveniently situated, and the most accurate, per- 

 haps in principle, of any hitherto constructed,) the obser- 

 vations are repeated several times in pairs near the meridian, 

 reading the divisions at both extremities of the air bubble 

 on the scale of the level each time along with the verniers 

 or microscopes. When there are three verniers and about 

 six observations made, it is advantageous to have a simple 

 and convenient method of registering the observations, 

 taking the means, and allowing for the effects of the level. 



The value of the divisions of the level is generally got 

 from the maker, or it may be readily found by an instru- 

 ment called the level trier, constructed expressly for this 

 purppse. 



If the observer has not had these communicated to him, 

 or if he wishes to satisfy himself with regard to the accuracy 

 of the values given to him along with the instrument, he 

 may either ascertain these by the circle itself, when the 

 verniers or reading microscopes are competent to the 

 purpose, or he may have recourse to the following methods, 

 which, in the course of my experience, I have found very 

 convenient. 



1 . Put up the usual levelling rod of the best construction 

 truly vertical, at such a distance from the circle as may be 

 most convenient, though somewhat considerable. 



2. Set the level exactly in the direction of two of the feet 

 screws, or one perpendicular to the line joining the other 

 two, when there are three ; clamp the verniers, and direct 

 the intersections of the cross wires of the telescope to the 

 mark on the sliding vane, which must be moved up or 

 down till an exact coincidence takes place. 



3. By turning one of the feet screws cause the bubble to 

 move through a given number of the divisions of the scale, 



