156 MEASUKEilENT OF AX^ ARC ON THE 



meter being divided into sixty parts, each part is 

 therefore two seconds of the circle. 



A number of experiments have been made for de- 

 termining the error of the semicircle, and to ascer- 

 tain the place of the fixed wire in the horizontal 

 microscope, so as to divide the error. It has ap- 

 peared in the event, that the telescope being in its 

 right position, (that is, when the limb and micro- 

 scope were on the left hand,) and the fixed wire 

 placed at Zero on the semicircle, when the circle 

 or limb of the theodelite was turned ISO" in Azi- 

 muth, ^nd the telescope turned over, the fixed M'ire 

 was then distant from Zero on the opposite part of 

 the arc by a mean of a great many observations 

 2' 5i\ the half of v/hich is therefore the error. 

 This half was carefully set off from Zero by the 

 moveable micrometer wire, and the fixed one 

 brought to coincide with it. On the right appli^ 

 cation of this error, there will be 1'28", 5 to add 

 to the elevations and subtract from the depressions. 

 The observations for determining this quantity 

 were repeated at different times, and under the 

 most favourable circumstances ; the adjustments of 

 the whole instrument being frequently examined, 

 and the level applied to the telescope, reversed at 

 most of the observations. For the line of collima- 

 tion, as these corrections depend on having a well- 

 defined o])ject, I fixed a bamboo upwards of a mile 

 distant from the observatory tent, and tied round 

 it several narrow stripes of black silk, one of which 

 was near the horizontal wire when the axis of the 

 telescope intersected the staff after being brought 

 to a level by the bubble. Then the instrument 

 being adjusted, and the telescope directed to the 

 bamboo,' being perfectly level, and the wire of the 

 micrometer in the piece brought to the intersection 

 of the cross wires, the angular distance to the 



