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. xin, 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 station errors. if. : Pe nat 
Ul. 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 
us investigation, all taken, with but two or three exceptions, 
rom my former articles, Column fourth shews the Jirst 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 incloded. interval, and. this. happens at 
