312 



Spirit level 

 Tnuch more ac- 

 curate. 



PLUMB LINE AND SP. LEVEL. 



point. Though thefe eight feconds meafure only .001 59 parts 

 of an inch, yet an experienced obferver will eftimate with 

 confiderable exa&nefs when the parts of the point feen on 

 each fide of the fhadow are equal. Still it is not improbable, 

 but a fecond or two may fometimes efcape his notice on fo 

 final 1 a fcale. 



The fpirit-level has the advantage of fo large a fcale, that a 

 fecond may be divided by the eye into many equal parts, and 

 it is fo eafy in its application, that a few minutes only are fuf- 

 ficient for determining the pofition of the axis of an agrono- 

 mical inftrument, to lefs than a fingle fecond, as appears by 

 the following obfervations. 



Obfervations of 

 the level, 



Pofition of the Axisfheton by the Spirit-Level. 



By the end x. 



l.lo 



1.65 



2.1 



1.7 



1.6 



By the end y. 



1.7 



1.85 



2.2 



1.77 



1.6 



x+y + 2 — 1.72 

 x+y-r- 2 = 1.75 

 x+y + 2 =2.15 

 x-f-y-r2 = 1.7 3 

 X +V -r 2 = 1.6 



Eaft end of the axis dips 



1.79 



*r«re fairly taken Thefe ten obfervations are independent of each other, they 



and reported. were all taken at the fame time, and none were rejected to 



make a (how of precifion, inftead of (hewing the truth. 



Other levels of I have had other fpirit-levels ground by the fame ingenious 



Troughton 2,rt\(i * wn i cn were as good as this, though one of them wis 

 equally accurate. ... 



filled with fpirits of wine. 



* Mr. Edward Troughton, Fleet-Street, London. He in gene- 

 ral ufes ether. 



XV. Defcription 



