Trigonometrical Survey. 475 
made to Mr. Ramsden in the foregoing winter, to consider of 
some means by which' their inclinations might be obtained. 
He therefore applied an arch S to the side of the transit tele- 
scope, as exhibited in Tab. XLIII. which he divided into half 
degrees ; and opposite to this he placed a microscope T, with 
a moveable wire in its focus, by means of which, and the mi- 
crometer of the telescope, an angle could be taken. 
On the first convenient opportunity after the arrival of the 
apparatus, we determined the value of any number of revo- 
lutions of the micrometer-screw in parts of a degree, by the 1 
following method. 
At the distance of an hundred feet from the transit, a picket 
was set up, on which a dot was made with chalk, and the in- 
strument being adjusted, was moved by the finger-screw till 
the edge of the micrometer-wire touched some prominent part 
of that mark. The wire in the focus of the microscope was 
then made to bisect a dot upon the arch, and the telescope 
moved in the vertical till the next dot was bisected, by which 
the instrument had described half a degree upon its axis, and 
the micrometer-wire was afterwards moved till it touched the 
same part of the chalk mark, the revolutions being counted, 
which were consequently equal to thirty minutes. This ope- 
ration was repeatedly tried, with a picket placed from one to 
six hundred feet successively from the telescope, the runs of 
the micrometer-screw being in each case nearly the same, as 
indeed they ought to be according to theory. 
The number of revolutions equal to 30' was found, from a 
mean of these trials, to be 12^^. 
Having determined this, the chains A and B were compared 
with each other, when they were found to have the same dif- 
ference of lengths as when measured by Mr. Ramsden. 
