106 Dip and Variation of the Magnetic Needle. 



Herschel is chairman. The committee urge the importance of 

 magnetic surveys of the surrounding districts in connection with 

 the observations at central stations. " By this means alone can, 

 the amount of station error for each element at the central sta- 

 tions be ascertained; by which is meant, all that part of each re- 

 solved element of the magnetic force, which, not being partici- 

 pated in by the surrounding district, must be attributed to attrac- 

 tions merely local and accidental. Without such surveys, this 

 error cannot be even approximately fixed." (See this Journal, 

 Vol. xLii, p. 154.) 



With regard then to the differences shewn in the last column 

 of my four tables, they are all affected by the errors of observa- 

 tion, which however, are believed to be generally quite small, 

 probably seldom exceeding one or two minutes ; they are also 

 affected by the errors of the hypotheses by which they are com- 

 puted, yet it is believed that at few stations do these errors exceed 

 two minutes. The principal part of these differences consists in 

 the statio7i errors. 



11. Variation of the Magnetic Needle. 



Although the magnetic chart contained in Vol. xxxix, p. 41 of 

 this Journal, is thought to give the variation of the needle with 

 sufficient accuracy for common uses, yet for purposes of science it 

 is desirable to attain the greatest possible precision. I have there- 

 fore instituted a careful examination of all the observations con- 

 tained in my two former articles, with such additional observations 

 as have since come to my knowledge. Of the latter class, the 

 most important are those which are given by Prof. Locke in his 

 Report on the Mineral Lands of the United States, Executive 

 Documents, 1839-40, Vol. VI, No. 239, p. 134. 



The first enquiry respects the present annual motion of the 

 needle. The following table exhibits the principal materials for 

 this investigation, all taken, with but two or three exceptions, 

 from my former articles. Column fourth shews the^rs^ and last 

 observations at each station employed in determining the annual 

 motion which is shewn in column eighth, and column sixth shews 

 the number of observations. When but two observations are em- 

 ployed, the annual motion is found by dividing the difference of 

 the observations by the included interval, and this happens at 



