PLUMB LINE AND SP. LEVEL. 309 



added diftilled water to the refidue. After agitating the mix- in excefs of 



ture it was filtered. The liquid thus obtained had a flight acid * 



role colour, and poffelfed all the properties of the cobaltic nation of oxl(ic 



acid. of cobalt and ar- 



This acid when mixed with alcohol afforded an abundant %£** gjjj 

 precipitate, and the precipitate when collected prefented all precipitablity, 

 the phenomena exhibited by the concrete cobaltic acid. and . a11 th f f rc " 



r * , pertics of Mte 



It appears therefore certain from the refults I have here f up p fed cobaltic 

 detailed, that the fo called cobaltic acid has no exiftence ; acid « 

 that the fimple combination of arfenic acid with oxide of cobalt Inference. 

 has led Citizen Brugnatelli into an error, becaufe this com- 

 bination with excefs of acid, is the fubftance obtained from 

 zaffre by the methods defcribed in this memoir. 



XIV. • 



On the Plumb Line and Spirit Level, by Ezekiel Walker. 

 Communicated by the Author, 



Lynn Regis, March 18, 1802. 



HEN it is conlidered that the accuracy of aftronomical Importance of 

 obfervations depends greatly upon the exact manner of ad- tl "; ad J uftment 

 jutting the inftruments, any improvement in the method of 

 making thofe adjuftments may not be thought unworthy of a 

 place in a philofophical Journal. 



The plumb-line and fpirit-level are much ufed by afrrono- pi um b_j; nean j 

 mers, but from the great improvements made in the con- fpirit-level. 

 ftruction of the latter, the plumb-line feems to lofe that pre- 

 ference which it had fome years ago. 



Mr. Ludlam fays *, " I believe the fpirit-level cannot af- common fpirit 

 " certain the pofition of the axis" (of an inflrument) '* nearer level faid not to 

 " than to a minute if fo near." And when I firft began to ex r cecd f* pre " 



& cilion of one 



life the fpirit-level for determining the pofition of the axis of a minute, 

 tranfit telefcope, I found it nearly in the fame imperfect flate 

 as mentioned by Mr. Ludlam : I was therefore very defirous 

 of having fome better method of making this adjuftment. 



After having thought much upon the fubject, I fixed on , .. 

 having a plumb-line fufpended from the object end of the tranfic inftru- 



ment. 

 * Aftronomical Obfervations, page 42. 



telefcope, 



