SQ^ ASTRONOMICAL CIRCLE. 



Decl. circle. The declination circle, 30 inciies in diameter, is composed 



of two complete circles; the conical radii of which are inserted 

 at Iheir bases in an axis about twelve 'inches long, leaving suf^ 

 ficient space between the limbs for a telescope Sy feet long, 

 and an aperture of 2| inches, to pass between. The two cir- 

 cles are firmly united at their extreme borders by a great num- 

 ber of bars, which stand perpendicular between them; the 

 whole of which will be readily understood by referring to the 

 figure. The square frames, which appear as inscribed in the 

 circle, were added to give additional firmness to the whole. 



The circle is divided by fine lines into 5' of a degree; and 



subdivided into single seconds l)y two micrometer microscopes, 



the principles and properties of which are now too well known 



to require any particular explanation. 



Greatimprove- At the time these observations were made, the microscropes 



ment that the were firmly fixed opposite to the horizontal diameter; but 



Se^'^crable of ^^'^*^'" ^ considered that, by continuing the observations, the 



having their error of division would never be diminished, I suggested to Mr. 



position varied Xroughton the possibility of giving: a circular motion to the 



by a circnbr .^ , , r ,- 



motion of the microscopes; though I confess with very little hope, that the 



diameterwhich thing was really practicable in an instrument previously con- 

 * structed on other principles. Mr. Troughton approved of the 

 idea, and executed it in a very ingenious manner. His talents, 

 as an artist, are too well, known and too highly appreciated, to 

 stand in need of any praise from me; yet I should consider 

 myself as deficient injustice, if I did not endeavour to call the 

 attention of the reader to the skill and ingenuity, which have 

 been employed not only in this very important alteration, but 

 in every C(^ntrivance that is peculiar to the instrument, which 

 is the object of our present consideration. 



These microscopes can now revolve about 60" from their 

 horizontal position ; and it is easy to comprehend, that, by this 

 vali'^able improvement, all errors of division may be completely 

 done away, without any of the manifest inconveniences of the 

 French circle of repetition ; which, though a very ingenious 

 instrument, and admirably adapted to some particular opera- 

 tions, will, I think, never be adopted for general use in our 

 observatories. 



TUeplumb-line The plumb-line, a very material part of this instrument, is 

 suspended from a small hook at the top of the tube at the left 



hand 



