26 W. A. Norton on the Variations of the Declination 
Anr. IIL—On the Periodical Variations of the Declination and 
Directive Force of the Magnetic Needle; by W. A. Norv 
Professor of Civil Engineering in Yale College. 
(Continued from vol. xix, page 211.) 
Iw the calculations made on pp. 207, 208, of the relative effects 
of the ecliptic and radial currents, no account was taken of the 
possible effects of residual currents, that is, of the gradually st 
siding currents which may succeed those which are directly de: 
veloped by the sun’s action. I now propose to inquire into th 
nature and extent of their action, to make due allowance for 
ination. But I would previously remark that there is no occasie 
to distingnish the action of the primary from that of the second+_ 
ary ecliptie currents, so called, at the equinoxes, in considering 
the entire effect in the interval from 6 a. m. to noon or from nod 
to 6 Pp. M. (See -p. 205.) It is only when we are comparing 
hourly variations that this distinction need be made. a addit 
to this qualification of certain statements made o 
should be observed (althou igh the correction is of little conse 
quence) that the oyaional current, previous to 6 a. M., is in 
cs tds inclined to the meridian at the two equinoxes. 
If we confine our attention to the entire semidiurnal interv 
just ebtioned then the ecliptic and radial currents tend, bot 
the forenoon and afternoon, to deflect the needle in the same 
of the ecliptic currents at the equinoxes; then taking the om 
from Table I, (p. 194,) we have, for the forenoon, r+e=8’ 
r—e= 5-67, and therefore *= 7-16 and wed For 
afternoon, r+e=7"89, r—e=2"78; hen = 5" 
e=2"55 he two values of r differ from dah “echar becaus is 
of the action of residual currents. As a first, approximation t 
currents, let R= diff. of effects of the primary radial eurrents se 
cirenlation at the beginning and end, respectively, of either the 
forenoon or the afternoon interval, and ¢= effect of the residue 
all the radial currents developed during either interval ; 
R+2=7'16, R—r=5'33, R=624, 2=0°92. Ma king th 
same calculations with the data from Table II, (p. 208,) 
have, for the forenoon, r=7/59, e=1"91; for ‘the afternoot, 
7r=4'50, e=2'-03: also R=6'-04, c=1'-54. “a 
197. The average of the two values of ¢ found above, from 
Table I, is 2’-02. The difference of the two ii ons 
only 0-05. 
