bis 
A. D. Bache on Declinometer Observations. — 208 
and the same holds yout for the morning epoch of th 
declination. The afternoon epoch, however, occu 
hich are most constant 
during the year, are those of the western elongation ae: of the 
morning mean declination. The greatest difference between any 
month and the mean of all the months, is 31 minutes in the 
former and 28 in the latter. 
To exhibit the features of the diurnal variation and its annual 
he magnetic surface is formed by contour lines 05 apart. The 
curves in dots (....) are lines of mean position. Those in 
ashes (-- --) are eastern deflections from the normal position 
and the full lines are western deflections. This Diagram and 
the table from which it is deduced are immediately applicable to 
the practical problem of furnishing the correction to be applied 
to a single observation, made at any hour of the day and month, 
to reduce it to its mean value. It also renders unnecessary the 
development of the annual variability of the coefficients in the 
analytical expression. The diagram distinctly exhibits the diur- 
nal minima and maxima, the former coms by a valley, the 
latter by a ridge, on the ‘magnetic surface 
ext, the magnitude of the diurnal range is studied. The 
rb table exhibits the amount of the deflection at the east- 
ern elongation and the diurnal amplitude of the 
asiostion for each month in the year derived from the equations. 
Deflection at [Diurnal Deflection at Diurnal 
E. Elong.|W. Elong.) range. E. Elong. W.Elong. Tange. | 
any, = Orde “43°52 | 5°98 || July. —5°58 | +546 | 11704 
Feby. ~2°64}+3°11 | 5°75 || Aug. -5 79 | 46°36 | 12°15 
March, -8 73 | 44°03 | 7°76 || Sept. ~4-71 | +5 -60 |} 10°31 
April. -4-02 | +528 | 9°30 || Oct. 218 | +8 -23 41 
May. —4°89 | +516 | 10 05 || Nov. -1°92 | +2°85 | 4°77 
June 5-26 | +5 06 | 10 32 ll Dec. -1°65!+8-14] 4°79 
e diurnal range for the summer months is 10-45, for winter 
months 5/56, and for the whole year 7°89, all corresponding to 
an epoch about a year and a half removed from the epoch pra a 
ii ae of the solar perio 
he numbers denoting the diurnal range, exhibit three re- 
markable —- namely, the maximum value in August, the 
sudden falling off in September and October, Cae HH) and 
the minimum value in November and December. In other re- 
spects, a Wy on is regular. The curve is nee e crested, 
Moe pei in the eastern, as well as the western deflections 
en viewed separately. This is of special im — as ib 18 
probable that Rg aah of th ese. separate curves at other 
