4 16 An Account of the Measurement 
of the screw should be exactly a minute. On trial, he found it 
nearly a second short ; for which reason, he divided the head 
into 59 parts, and called each of them a second. I think it 
proper to repeat the observation, that the two arches were 
measured with the greatest care, because it admits of the remark, 
that every space subtending 5' was measured with the same part 
of the screw, beginning very nearly from 9 on the index. This 
instrument will, at a future period, probably pass into other 
hands ; it may therefore be right to state, that 1 found, from an 
examination of the screw, an error of nearly 1", in the part 
contained between 17 and 19 on the index, arising from a small 
notch which, with a magnifier, I could plainly perceive on one 
of the threads. As it cannot but be the general wish to have 
some evidence of the accuracy with which this sector has been 
divided, and also how far I have succeeded in the performance 
of what is now under consideration, a table will be given, in 
which the value of every 5', in the first degree on each side 
zero, will be found in revolutions and parts of the screw. 
Having towards the end of June found my observations suffi- 
ciently numerous, and apparently sufficiently accurate, from the 
regular differences subsisting amongst them, I took down the 
sector, and, with every thing belonging to it, repaired to Clifton, 
the northern extremity of my arch. The instrument arrived 
there in safety, on the 20th of July ; and, as the direction of the 
meridian had been previously determined, the instrument was 
immediately set up, and made ready for use. 
At this station, Laughton spire afforded me an excellent mark 
for adjusting the instrument in the plane of the meridian. The 
bearing of it is T 56' 12" south-west; and, from my being able 
to see it in the observatory, without rolling up either of its 
