C. A. Schott—Magnetic Declination in the United States. 25 
Art. IV.—Abstract of results from a new discussion of the secular 
change of the Magnetic Declination in the United States, and some 
adjacent places in North and Central America ; by CHARLES A. 
ScHort, Assistant U. S. Coast Survey. 
(Read before the National Academy of Sciences, Nov. 4, 1874.) 
[Communicated by permission of CaRuILE P. PATTERSON, Superintendent U. S. 
Coast Survey. |* 
the magnetic declination (variation of compass) for a certain 
epoch, or date of publication, and its rate of change. 
0 briefly recapitulate the formule employed, let 
D=6-+r sin (am+e)+r, sin (a,m+e,)+ 
express the magnetic declination at any time ¢, positive when 
west, hegative when east of north; also, let m-=number of years 
(and fraction of a year) from thé adopted epoch ¢, or m=t—1850. 
a a, . . factors depending on the adopted periods, 
acs BE Oy EO 
cae oe ee 
The quantities 0, a constant representing a mean declination, 
"7... . parameters, and ec ¢, ... . epochal constants of the 
periodic terms, are to be determined from the observations at 
any one place by the application of the method of least squares, 
in order to satisfy the condition that the sum of the squares 
of the residuals of the observed and computed declinations 
shall be a minimum (zA*=a minimum). For this purpose put 
0=d,+«, where 6 ,=an assumed approximate value 6, and z a 
Correction to it; also, 
recose=y and rsin c=z, 
then the conditional equations will take the form 
0=0,—D+a+sin am.y+tcosam.z+... 
The paper in full will be published in Coast Survey Report for 1874. It is 
an of the article which appeared in this Journal, vol. xxix, Art. XXIX, 
’ 0. 
