A. D. Bache on the Horizontal Component of Magnetic Force. 265 



The secondary minimum is reached about 8^ SO™ p. m., with a 

 comparatively small range. 



The mean value of the force is attained about 7^ 55™ a.m., and 

 again about 1'' 55" P. M., with considerable regularity ; it is again 

 reached at efh, and at 11| p. m., though with less regularity. 



At Toronto (see vol. II. of the Toronto Observations) the 

 diurnal variation of the horizontal force has a principal maxi- 

 mum at a little after 4 P. M., and a principal minimum at 10 or 

 11 A. M. ; the secondary maximum occurs about 6 A. M. There is, 

 therefore, this specific difference in the diurnal motion at these 

 two stations: in that at Philadelphia the morning maximum is 

 the higher of the two, while at Toronto it is the afternoon max- 

 imum. The difference between the two maxima, as shown above, 

 is ahnost nothing in the maximum year 1843-44, but increases 

 before (and after) this epoch in proportion to the interval. 



At Toronto the daily range seems to be slightly greater. The 

 secondary minimum at Toronto occurs about 2 or 3 A. m., or 

 about t> hours later than at Philadelphia ; this is a second though 

 less significant point of difference. 



The minimum daily range occurs in 1843-44, and is then less 

 than one half of what it was in 1840-41. 



The equation for the diurnal range in scale divisions gives the 

 following results : 



R=+19-68-3-78 («-1843)+ 2-77 (^-1843)' 

 it represents the observed values as follows : 



The minimum range, as given by the formula, is m September, 

 1843. In part I of the discussion we found the mmimum range 

 of the declination in May, 1843, and the minimum from the dis- 

 turbances of the declination in August, 1843. 

 investigation of the eleven (ten ?) year inequality in the disturbances of the 



horizontal magnetic force. 

 , In table VI the number of disturbances in each month has 

 been given as found from the observations ; these numbers are 

 however not directly comparable with one another, first on ac- 

 count of some omissions in the record, and secondly on account 

 ?f the change from a bi-hourly to an hourly series. lor any 

 incomplete month the number of disturbances for the whole 

 °iOQth is obtained by simple proportion from the number during 



^- Jock. Sci.-Secohd Sebos, Vol. XXXIV, No. 101.-Skpt., 186a 



