VOL. LXXXIII.] PHILOSOPHICAL TRANSACTIONS. 311 



ance of being more to the left than they ought to be ; and thence would give 

 the measurement too small, and that in an equal degree in each ; so that the 

 sum of zenith distance given by one microscope, and of altitude by the other, 

 would thereby be less than QO degrees, by just double the error. And if the 

 axis be moved towards the left, the contrary would be the result ; the sum 

 would exceed QO degrees by just double that quantity. Hence the difference 

 from go degrees, at the same time that it gives a mean between the 2 readings, 

 would reduce the error or deviation of the axis to nothing. 



I may be supposed partial to an idea which I have long entertained ; but I con- 

 fess I should very strongly recommend the having an instrument of this nature, 

 though more perfect, in every observatory ; I mean a transit instrument, on 

 stone piers, with a suitable circle and microscopes ; that whenever we observe a 

 meridian passage, we may, at the same time, measure the exact altitude, or 

 zenith distance of every object seen. The being obliged, in the common way, 

 to have recourse to 2 different instruments, occasions the zenilh distances to be 

 much less frequently observed, than it is to be wished they were. It is true the 

 British catalogue was, for the most part, deduced from observations with a qua- 

 drant alone ; and so was Mayer's. But though labour and patient perseverance 

 may enable an observer to allow for any deviations in the limb, a quadrant is at 

 the best but an imperfect instrument for right ascensions. 



In observing, I always study to be as much at my ease as possible: and there- 

 fore I always sit, and use a prismatic eye-glass. To avoid touching the instru- 

 ment itself, or even the stone on which it stands, I have 4 upright poles from the 

 floor to the roof, with cross braces on a level with the bottom plate of the instru- 

 ment ; against which I may lean, while I observe, or when I handle any part of 

 the instrument. These I find to be of great comfort and use. Against 2 of the 

 poles I hang a curtain occasionally, to keep off the sun, or to lessen the false 

 light when I observe a star in the day. 



Indeed, on the whole, this instrument itselfis capable of doing a great deal of 

 good work ; and convinces me fully, that one between piers would be highly ad- 

 vantageous to astronomy. As a transit, mine is perfect, so far as that size per- 

 mits : indeed it is in fact to all intents a transit-instrument. And for altitudes ; 

 since the readings are totally independent of the circle, though we have it in our 

 power to re-examine the microscopes by the plumb-line between each observation, 

 if we please ; we find there is no occasion for it. In that respect, it has the ad- 

 vantage over a quadrant. No force is used in setting this instrument: the whole, 

 from its form, is counterpoised in itself: there is no more probability of de- 

 ranging it in altitude, than in azimuth: and therefore, ail we have to do in 

 actual observation beyond a common transit-instrument, is, to bisect the star as 



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