376 SURVEY OF THE COAST 



the axis out of the circle, to adapt this latter to the engine, 

 and consequently it is not absolutely certain that the axis be 

 screwed again exactly in its position, ^'c. 



The dividing engines have all a centre pin, to which the 

 instrument is centred. These pins are either tixed, or 

 changeable, for different diameters of axes, suited to the usual 

 and common instruments. To adapt any instrument having 

 a central aperture for an axis different from these, a collar is 

 turned, having its outer diameter to fit this aperture, and its 

 central hole the centre pin of tlie engine. But it is evident, 

 that the concentricity of these two circles is not always cer- 

 tain, and that in the last turning above mentioned, the centre 

 may have been displaced from the centre of the aperture, 

 which receives the axis, and which was of course not turned 

 with it. 



The division being completed, the axis is again screwed in 

 its place, and the adjustment of it is to be made, both with 

 respect to its concentricity and its perpendicularity to the 

 circle. The reading of opposite verniers or microscopes 

 must indicate the corrections which are necessary. To 

 give the axis its proper position, it is necessary to raise three 

 burrs with a punching tool at such places as appear to re- 

 quire it. By repeated trials of tills kind, the axis is again 

 centred, and this operation is to be discontinued when a suf- 

 ficient approximation is obtained. When an instrument has 

 to be removed to a considerable distance, these burrs may 

 wear out by the greater pressure they sustain, and the in- 

 strument may become eccentric by it. 



All this troublesome operation, and the disadvantages at- 

 tending it, may easily be avoided, if instead of the pin or 

 axis, the dividing machine had in the centre a circular hole 

 of about five inches diameter, and of sutticient depth to ad- 

 mit the lower parts of the axis of any instrument, so that the 

 instrument could be placed upon the engine, together with the 

 axis, in the same situation as it came from tlie last turning 

 and rubbing, without any dismounting whatever. To ease 

 the approximate placing of the circle on the engine, a num- 



