40 W. A. Norton on the Variations of the Declination 
during ae de months when the sun is north of the equator, ¢ 
from April to September, than in the opposite season.” 
We fee seen (p. 38) that there are two causes in operation 
tending to produce a diurnal variation of the vertical force ; in the 
ronto the tendency to an increase, by reason of the eae clit a 
rents, prevails early in the forenoon over the tendency toa 
“Sgeencon the other tendency prevails and a secondary minimum 
uts. The range from 6 a.m. to 6 P.M. is entirely due to the 
residual’ currents. 
et c= effect on the vertical force of the difference of intensit 
of the ecliptic currents on opposite sides of the zenith, at noo 
c’= same at 6 a.m. or 6 P. M.; s= effect of sum of residual cut 
rents at noon, resulting from forenoon excitation, or of residt 
currents at 6 Pp Iting ffom afternoon excitation, m= rise 
vertical force in th | from 6 a. mM. to noo m’/= same 
6 P supposing the residual oo" in ac 
tion at noon to continue withont ¢ pepetion until 6 P 
s—(e’—c)=m, s+(c—c)=m’ ; ;8=—9 
Or supposing the residual currents at noon to pad off 
ing the afternoon, 
+(e) Ve e’—c=m'; s=m-+(ce’—c) . . . (24). 
The following Table contains the values of s and (c’—c) f 
each month in the year, com isi from equations (23) and (24), 
and the means of these value 
Jan. | Feb. | March.) April.| 3 es \ Win ne. |July.|Aug.) Sept. | 
eo 4 | See Pe ale | O54 45 1 3 | 165 is 
we 9 Ob oar hy ‘ 2412 3 
Means, | 10:5; 13:5 | 14-26 | 17-25) 1425 | 6-75 | 18 | 15 24°75; 225 
c!-c 4 105 | 105} 185 | 165 Lh 101) -25:, -2 
cl-c | 1 1 20 22 28 : 26 | 20 
| Means, | 7:5! 9°5 } 15°25 | 16:25! 23-25 | 18:75 | 20 15 | 575! 55 
The means, which answer approximately to the supposition — 
that the residual currents in action at noon are reduced in inten- 
tical force at other stations, we find that there is a considerabl 
diversity. For example at Philadelphia, the maximum is generally 
at 2 p.m., and the force decreases from this hour to about mid- 
night, then increases until 2 a. M., and decreases again until 4 
less 
after that hour. At Toronto this falling off in the angme 
amounts to the production of a secondary minimum. | 
