Ill 



The mean value of all the observations may then be determined and the value of 

 one division of the level expressed in minutes or seconds of arc, as the case may be. By 

 this same process we may find, as already shown, the radius of curvature to which the 

 interior surface of the glass tube has been ground. 



For spirit levels having no scale graduated on the tube the german silver scale 

 graduated to 20ths of inches and attached to the apparatus is to be used as shown in 

 the cut. For testing the levels of an Engineer's Instrument as already indicated it is 

 not necessary to remove them, but the instrument entire can be placed directly on the 

 Trier. 



The Instrument should then be leveled by its leveling screws, and the level to be 

 examined should be brought parallel to the center line of the bar by the use of the 

 tangent screws. (This saves much unnecessary wear on the fine micrometer sere w. ) 

 This done, proceed in the same manner as explained for the unmounted spirit level. If 

 we have the case of a wye level bubble that needs examination, the telescope and level 

 tube may be removed from the wyes of the Instrument and placed on the wyes of the 

 Trier, as shown in the cut. The forked arrangement at the pivotal end which can be 

 clamped at any convenient place on the projecting rod, will tend to steady the telescope 

 and prevent it from falling while the level is being tested. 



This ability to readily and speedily examine the levels of an instrument without 

 detaching them and thereby deranging instrumental adjustments will, we think, be 

 quickly appreciated. 



As already indicated above, a level should be tested forward and backward, and, in 

 case of very fine spirit levels, such as are used for measuring angles, this should be 

 repeated a number of times so as to get a considerable number of observations from 

 which to deduce the mean value of one division in seconds of are. In case of a level 

 provided with an air chamber reading to single seconds of arc as a rule, it will be 

 found necessary in making the test to keep the bubble of a constant length (a good 

 length is two-fifths the length of the tube, excluding that of the air chamber.) 



We here insert specimen sets of observations made upon levels taken from stock, 

 and such as are used in our engineer's wye level. 



Ten divisions of micrometer screw cor- 

 respond to 19.2 seconds of arc, therefore 

 one division of scale = ^ = 4.4 seconds 

 and one division on bubble tube (tenths of 

 an inch) = 8.8 seconds. 



Five divisions of the micrometer screw 

 = 9.6 seconds of arc, therefore one divi- 

 sion of level = f| = 6.4 seconds of arc. 



