4i HEAStJRE vlF ▲ DEGREE OK THE CtfROJfANDEt COAST- 



tliat means I could discover the smallest paralkx. If it ad-* 

 mitted being brought nearer to the axis, it was done; but I 

 found IVora experience, that it was more eligible to leave the 

 wire at a sensible distance, than to bring it very near. Hav- 

 ing satistied myself in this particular, I examined with the 

 microscope again in front, moved the wire freely in the ver- 

 tical plane, and then bisected the dot. The telescope was 

 then moved, so that the wire was brought over the dot zero 

 on the arc, and the same precaution used with respect to 

 the wire moving free of the arc; and here, as well as above, 

 I found it best to allow a sensible distance between the wire 

 mid the arc. 

 Adjustment of Xhe microscope by which the Upper dot in the horizontal 

 t microscope, ^^j^ ^^ examined being fixed by the maker, the axis of vision 

 is of course at right aiigles to the vertical plane, and will 

 meet that plane in the centre of the axis; but the lower* 

 microscope is movable, and requires care to fix it so as to 

 have the wire in the axis of vision, and be free from the ef- 

 fects of parallax ; this I have done by moving it along the 

 brass plate in front of the arc, till the wire appeared free 

 from curvature, and then adjusted the dot. In these late 

 observations, I have generally made the final adjustment by 

 the light of a wax taper, for the wind being sometimes high 

 and troublesome, I found there was much irregularity in the 

 observations, until I adopted that method. I therefore 

 closed the doors and windows of the observatory tent, so as 

 to have a perfect stillness within. The distance of the wire 

 from the axis and the arc is likewise better defined by a taper 

 by noticing the shadow jn moving the light to th^ right and 

 left. 

 Fixing the in- In fixing the instrument for the star, great care was taken 

 »tr»meat. ^^ have it placed in the meridian, which was done by a mark 

 at near the distance of a mile (generally one of my small 

 flags), the polar star having been previously observed by 

 the large theodolite for this purpose. The telescope was 

 then moved in the vertical, till the wire of the plummet was 

 at the nearest division on either limb to the zenith distance 

 of the star, which could always be nearly known. The mi- 

 crometer, having been put to zero, was firmly screwed, and 

 the dot on the limb carefully bisected, the instrument was 



turned 



