PLUMB LINE AND SP. LfcVEL. 311 



In confequence of an accident which happened to my fpi- Plumb line re- 



rit-level, I was under the neceflity of having recourfe once 



more to my plumb-line. 



But Mr. Nicholfon's paper on page 134, Vol. I. of his Sufpidon of 

 , . x , , i • -.i ir- flexure in the 



Journal, (quarto) made me look upon it with a very lulpi- w j re# 



cious eye. My having been aflured that my wire was of the 

 beft quality, did not remove thofe fufpicions which that paper 

 had raifed in my mind of its being crooked. At laft a method How verified. 

 of trying this by experiment occurred to me, which was by re- 

 verting the fides of the wire, thus : after having made the 

 wire to bifect the point near the eye end of the telefcope > I 

 turned that fide of the wire which faced the fouth, to face the 

 north, which was done very eafily, by reverting that piece of 

 brafs to which the wire is fixed at the top, and at the fame 

 time turning the plumb line half round the lame way. To my The error from 

 great furprize I found that my wire did not form an accurate flexure w * s fe_ 



. . J . vaa legends. 



right line by feven feconds, though it was loaded with a pro-, 

 per weight. 



I tried another piece of wire from the fame bobbin, which 

 deviated fix feconds from a right line ; I tried feveral other 

 pieces before I found one that flood the teft of this examination. 



But after the error of a plumb-line has been thus deter- Remedy. But 

 mined, it is as good as if it were perfectly ftraight. be'invIrbblTT 



Another inconvenience that attended my plumb-line was W. N. 

 this, — though the wire hung as near the point as poffible with- Difficulty of ab- 

 out touching, yet the microfcope could not be adjufled to fe J vin S *he 

 view them both diftinctly at the fame time. An accident, ftroke at the 

 however, put it in my power to remove this difficulty. Le- fame time ' 

 veiling the axis of the tranfit, one day, without any light in Removed by 

 the room, except that which came through the opening in ufin § tne madow 

 the roof behind me, I obferved the fliadow of the wire upon 

 the telefcope. By placing the microfcope a little on one fide 

 of the wire, I faw the fliadow very black and perfectly well 

 defined. The fhadow and the point being on the fame plane, 

 both obje&s were diftinctly feen with the fame adjuftment of 

 the microfcope. 



The length of this plumb-line from the notch in which the Eftimate error 

 wire reds at the top to the point is about 41 \ inches. The of adjuftment by 

 wire fubtends an angle of 24 feconds at the point, and the 

 point itfelf 32 feconds, confequently four feconds of the point 

 are feen on each fide of the fhadow when the wire bifecls the 



point. 



