[ 539 ] _ 
of the micrometer will fliew the difference of decli- 
nation of the ftars ; and if the times be noted when 
they pafs the horary wire N S the difference of the 
times will give the difference of their right afcenlion. 
For E W on account of the bar’s running along it, 
is parallel to tlie equator j and confequently N S 
which is perpendicular to it, reprefents a meridian or 
horary circle. And becaufe the two images of the fame 
ffar pafs the horary wire N S at the fame inftant, it 
follows that the centres of the two femicircular glaffes 
lie in the lame meridian, and confequently when A, B, 
the two contrary imao:es of the two bars are brought 
to tlie fame parallel of declination E W, the fcale 
will blew the difference of their declinations. And, 
for the fame reafon, the times of the images of the 
two bars oalTing the meridian wire N S will not be 
1 O ^ 
affedled by the feparation of the glaffes of the micro- 
meter, and confequently the difference of the times will 
give the difference of their right afeenbon. It will be 
eafiiy underbood that in performing the operations 
above deferibed, it will be neceffary from tihie to 
time to turn the ferewsof the rack- work which move 
the whole telefcope together. Thefe operations will be 
much facilitated and rendered more exa£f, if the tele- 
fcope be fupported by and moveable on a polar axis ; 
for the wires and micrometer may be thereby more 
readily brought into the requifite pofitions, and the 
turning of the telefcope about in order to follow the 
diurnal motion will not diburbthofe pofitions; which 
will afford this farther advantage to the obferver, of be- 
ing able to repeat the obfervations without lofs of time. 
If two additional horary wires F G, H I parallel 
to N S be placed near E and W the two extremi- 
Z z z 2 ties 
