4 Ona New Method of recording differences of Declination. 
‘ment within the above narrow limits, inasmuch as the. wider 
range of observation will include the more restricted. 
- When on a comparison of the work of two different nights, 
the discrepancies were reduced to the fraction of a second of are, 
it became manifest that the micrometer was (as built) incapable 
of measuring with certainty such minute quantities. 1 had 
neither time nor means to build a new and more perfect instru- 
ment, and finding the micrometer reliable for half a dozen revo- 
-lutions of the micrometer screw, a method of intercomparison 
e work of two different nights occurred to me, which with my 
defective micrometer would test in the most absolute manner the 
powers of the new machinery. This was as follows, viz. 
The stars of the catalogue were observed during one night on 
the odd wires, 1, 3, 5, 7, and 9, leaving the spaces blank on the 
engraved record, which corresponded to the even wires 2, 4, 6,8 
and 10. Every thing remaining untouched, on the. following 
clear night the same stars were observed on these even wires, al 
thus a decade of dots was recorded, of which five were engraved 
on one night, and five others on the following one. . 
These records thus intermingled and interlocked, were now 
easily read by the micrometer, and falling as they did within the 
limits of reliable performance of the micrometer, the capacity of 
the instrument to repeat itself on different. nights was tested in 
the most absolute manner, no matter if. the stars observed com- 
prehended a zone of even 25° or 30°. _ It is proper to remark that 
in all these observations I was assisted by Mrs. Mitchel, the stars 
corded by another, no bias could possibly be given to the person 
ing... 
idiosyncracy, while the readings being made by one person and re- ! 
In our first experiment of interlocking observations, forty-eight 
c 
hours elapsed before it be 
ame possible to remove the metal plate — 
from the pier, during which interval of time there were constant ; 
changes in the atmosphere, with storms of rain and wind. It — 
was therefore with no small anxiety, that on closing the observar — 
tions the microscope was turned npon the decade of dots, the — 
work of two nights at intervals of forty-eight hours, 
amination, was in the highest degree satisfactory. The dots were _ 
placed with a precision and beauty perfectly astonishing, and no 
eye, thus far with the microscope itself, “wt been ahinia mark — 
the difference between a decade of dots: struck on two different — 
nights, and one struck all on the same night. . It did not require — 
the micrometer screw to decide the question as to whether the 
instrument had repeated itself on the two different nights. This 
was obvious froma mere inspection of the dots by the. micro- 
scope. In two instances on the extreme stars there was a slight 
