the ^Trigonometrical Operation. 145 
circle, having their extremities terminated in a kind of blunted 
triangular figure, forming the bafes of pedeftals whereon ftand 
the vertical microfcopes hereafter to be defcribed. The arms, 
together with the horizontal bar and braces carrying the tranfit 
teiefcope, are every where pierced, in order to leflen the weight 
without diminiftung the ftrength of the parts. 
Art. i g, Vertical Microfcopes. 
Two vertical microfcopes, diftinguiftied A and B, are ufed 
for reading off the divifions on the oppofite fides of the circle 
immediately under them. They are exactly of the fame con- 
ftru&ion, and the chief parts of that marked A are reprefented 
in their real dimenfions towards the left hand of Plate VI.; 
where, befide the general, may be feen particular plans of the 
Hides, and alfo that of the pedeftal, containing within it the 
gold tongue, with its axis and fcrews for adjuftment. Next to 
thefe plans ftand the elevation and optical lines, (hewing the 
pofition of the glafies with the magnified fcale at the bottom. 
Each microfcope contains two Hides, one lying immediately 
over the ether, their contiguous furfaces being in the focus of 
the eye-glafles. The uppermoft, or that neareft the eye, is a very 
thin plate of brafs, to the lower furface of which is attached 
the fixed wire, having no other motion than what is neceffary 
for adjuftment, by the left hand ferew to its proper dot, as 
hereafter to be explained. 
The fteel Aide immediately under the former is made of one 
entire piece, of fufficient thicknefs to permit the micrometer 
ferew, of about 72 threads in an inch, to be formed of it. To 
its upper furface is fixed the moveable wire, which changes its 
place by the motion of the micrometer head, feen in the plan 
and elevation towards the right hand. The head is divided 
Vol. LXXX. U m into 
