182 MEASUREMENT OF AN ARC ON THE 



ing free of the arc ; and here, as well as above, I 

 found it best to allow a sensible distance -between 

 the wire and the arc. i :)di oali: 



The microscope by which the upper dot in the 

 horizontal axis is examined, being fixed by the 

 maker, the axis of vision is of course at right angles 

 to the vertical plane, and will meet that plane in the 

 centre of the axis, but the lower microscope is move- 

 able, and requires care to fix it so as to have the 

 wire in the axis of vision, and be free from the 

 effects 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 ad- 

 justed 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 ob- 

 servatory 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 in moving the light to the 

 right and left. 



In fixing the instrument for the star, great care 

 was taken to 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 the-r 

 odolite for that 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 micrometer, having been put 

 to zero, was firmly screwed, and the dot on the 

 limb carefully bisected, the instrument was turned 

 half round ; the adjustment examined and correct- 



