430 Mr. J. Nixon on the Trigonometrical Height 



tion additive for the semidiameter of the wire. As the arcs are 

 not fixed exactly parallel to each other, the dot does not make 

 precisely half a revolution downwards on inverting the tele- 

 scope, and therefore cannot come to the previous perpendi- 

 cular distance from the axis of the cylindrical telescope by a 

 Quantity (too trivial, however, to be regarded,) equal to thi^ 

 istance multiplied by the versed sine of the deviation of pa- 

 rallelism of the arcs. It may be remarked also that when the 

 cross wires are properly adjusted, the perpendicular one, from 

 a defect in the stop, cannot be set quite parallel to either arc. 

 This was first noticed on observing the dip of the sea, the 

 horizontal wire being slightly inclined to the edge of the sea 

 when either arc was vertical; but the deviation is not of a na- 

 ture to vitiate the measurements. The reversing point* of 

 neither of the great levels, but particularly of that marked G, 

 proved so constant as heretofore; the seasoning which the 

 levels had gradually acquired from repeated shocks in travel- 

 ling, and by exposure to extremes of temperature, having been 

 disturbed by dismounting them on commencing the survey 

 and by frequent alteration of the adjustments, never in the 

 course of any campaign, but during the intervening winters. 

 As the fluctuation must have taken place chiefly on trans- 

 porting the sector from one station to another rather than at 

 the time of observation, the angles cannot have been materially 

 affected in consecjuence. A register of the reversing points 

 for each level is subjoined in a notef. Generally the sector 



* That degree of the scale at which the middle of the bubble stands 

 when the axis of the cylindrical tube of the telescope lies parallel to the 

 horizon. 



t Dates. 



"August 



1829.< 



September 



October 



1830. <^ 

 U 



n 



November 

 'July 



1832..^ September 15 



(1° of the scale is about 2".) 

 The sector was supported either by solid masonry or bedded rock. 



