and Directive Force of the Magnetic Needle. 33 
same hour in the afternoon. The tendency of residual currents, 
so far as they may act, will be to make the minimum happen later 
in the forenoon, and the maximum later in the afternoon 
But the ecliptic currents play a no less important part than the 
radial, which we must proceed to consider. At noon, or near that 
i these currents will be perpendicular to the meridian, fe 
ont 6 a.m. and 6 p.m. their obliquity to the meridian wil 
“ its maximum, ‘These currents will always tend to Pie 
the horizontal force, and will act with the greatest effect when 
their obliquity is the least ; since their components in the diree- 
tion perpendicular to the meridian will then be the greatest, 
for the erie = that the currents themselves are then the 
most inter Their tendency therefore is to make the minimum 
of hcbawnadl force fall at or near the hour of noon, and the max- 
imum at about 6 a.m.and 6 p.™. Acting as they do, in conjune- 
tion with the vial currents, they will have the effect to make 
the minimum fall later in the. forenoon, and the poate maxi- 
mum later in the afternoon. The relative effect of the two sets 
of currents, it is reasonable to suppose, should however be differ- 
ent in different seasons, and hence the hours of maxima and 
minima Sats = as wer known to do, from one sea- 
son to anot 
To acer she comparative semidiurnal effects of the ecliptic 
and radial eurrents, denote them en eine by R and E, an 
aga soars of horizontal force from 6 a. M. to noon, and m= 
variation from noon to 6 p.m; pie m=RKR—E, m'=R+E; 
—m m = 
hd 
whence f=—5—s and R= ..-(20). It is to be observed 
that R is equal to the effect of 43 component, perpendicular to 
the meridian, of the resultant of the radial currents at 6 a. m., an 
nearly equal to that of this resultant itself; and that E is nearly 
equal to the effect of the ecliptic currents at noon. We here 
leave ont of view the residual currents. ‘To take them into ac- 
count let r= — ou horizontal force of residual radial enrrents 
at noon ; r’= same ye p.M.; e= effect of residual ecliptic cur- 
rents at noo n; = je e at 6 p. m.; & and /= effects of nocturnal 
radial and ecliptic ombeatiy respectively, in action at 6 a. Mm. (or 
eae of ~ ogee of them which passes off during the fore- 
n); ie proportion of residual and ge= the cea of 
colitis acu that pass off during the afternoon, t 
—E-r—etk+l, m'=R+E+?r’+pr+ge—¢ .. . (21), 
r=r’ (nearly) and e=é (nearly); £andZare emmparatively small ; 
the values of ; a and lie between o and 1, but are probably g greater 
ha ave then, ; 
m! —m ae 
2 
m-+-m pr+e'g—2 
ee hs a A pre g—®) ) 
Seconp Sertzs, Vol. fy No. 58.—July, 1855. 5 
(nearly), — 
